<?xml version="1.0" encoding="US-ASCII"?>
<!--XBRL Document Created with Wdesk from Workiva-->
<!-- -->
<linkbase xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <roleRef roleURI="http://questdiagnostics.com/role/BusinessAcquisition" xlink:href="dgx-20150930.xsd#BusinessAcquisition" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/BusinessAcquisitionNarrativeDetails" xlink:href="dgx-20150930.xsd#BusinessAcquisitionNarrativeDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/BusinessSegmentInformation" xlink:href="dgx-20150930.xsd#BusinessSegmentInformation" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/BusinessSegmentInformationDetails" xlink:href="dgx-20150930.xsd#BusinessSegmentInformationDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/BusinessSegmentInformationTables" xlink:href="dgx-20150930.xsd#BusinessSegmentInformationTables" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/CommitmentsAndContingencies" xlink:href="dgx-20150930.xsd#CommitmentsAndContingencies" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/CommitmentsAndContingenciesDetails" xlink:href="dgx-20150930.xsd#CommitmentsAndContingenciesDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/ConsolidatedBalanceSheets" xlink:href="dgx-20150930.xsd#ConsolidatedBalanceSheets" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/ConsolidatedBalanceSheetsParenthetical" xlink:href="dgx-20150930.xsd#ConsolidatedBalanceSheetsParenthetical" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/ConsolidatedStatementsOfCashFlows" xlink:href="dgx-20150930.xsd#ConsolidatedStatementsOfCashFlows" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/ConsolidatedStatementsOfComprehensiveIncome" xlink:href="dgx-20150930.xsd#ConsolidatedStatementsOfComprehensiveIncome" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/ConsolidatedStatementsOfOperations" xlink:href="dgx-20150930.xsd#ConsolidatedStatementsOfOperations" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/ConsolidatedStatementsOfStockholdersEquity" xlink:href="dgx-20150930.xsd#ConsolidatedStatementsOfStockholdersEquity" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/Debt" xlink:href="dgx-20150930.xsd#Debt" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/DebtLongTermDebtDetails" xlink:href="dgx-20150930.xsd#DebtLongTermDebtDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/DebtMaturitiesOfLongTermDebtDetails" xlink:href="dgx-20150930.xsd#DebtMaturitiesOfLongTermDebtDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/DebtNarrativeDetails" xlink:href="dgx-20150930.xsd#DebtNarrativeDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/DebtTables" xlink:href="dgx-20150930.xsd#DebtTables" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/DescriptionOfBusiness" xlink:href="dgx-20150930.xsd#DescriptionOfBusiness" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/DispositionAndHeldForSale" xlink:href="dgx-20150930.xsd#DispositionAndHeldForSale" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/DispositionAndHeldForSaleDetails" xlink:href="dgx-20150930.xsd#DispositionAndHeldForSaleDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/DocumentAndEntityInformation" xlink:href="dgx-20150930.xsd#DocumentAndEntityInformation" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/EarningsPerShare" xlink:href="dgx-20150930.xsd#EarningsPerShare" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/EarningsPerShareDetails" xlink:href="dgx-20150930.xsd#EarningsPerShareDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/EarningsPerShareTables" xlink:href="dgx-20150930.xsd#EarningsPerShareTables" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/FairValueMeasurements" xlink:href="dgx-20150930.xsd#FairValueMeasurements" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/FairValueMeasurementsNarrativeDetails" xlink:href="dgx-20150930.xsd#FairValueMeasurementsNarrativeDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/FairValueMeasurementsRecognizedAssetsAndLiabilitiesAtFairValueDetails" xlink:href="dgx-20150930.xsd#FairValueMeasurementsRecognizedAssetsAndLiabilitiesAtFairValueDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/FairValueMeasurementsReconciliationOfBeginningAndEndingBalancesOfAssetsAndLiabilitiesUnobservableInputsDetails" xlink:href="dgx-20150930.xsd#FairValueMeasurementsReconciliationOfBeginningAndEndingBalancesOfAssetsAndLiabilitiesUnobservableInputsDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/FairValueMeasurementsTables" xlink:href="dgx-20150930.xsd#FairValueMeasurementsTables" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/FinancialInstruments" xlink:href="dgx-20150930.xsd#FinancialInstruments" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/FinancialInstrumentsNarrativeDetails" xlink:href="dgx-20150930.xsd#FinancialInstrumentsNarrativeDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/FinancialInstrumentsSummaryOfFairValueOfDerivativesDetails" xlink:href="dgx-20150930.xsd#FinancialInstrumentsSummaryOfFairValueOfDerivativesDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/FinancialInstrumentsSummaryOfNotionalAmountsDetails" xlink:href="dgx-20150930.xsd#FinancialInstrumentsSummaryOfNotionalAmountsDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/FinancialInstrumentsTables" xlink:href="dgx-20150930.xsd#FinancialInstrumentsTables" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/GoodwillAndIntangibleAssets" xlink:href="dgx-20150930.xsd#GoodwillAndIntangibleAssets" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/GoodwillAndIntangibleAssetsDetails" xlink:href="dgx-20150930.xsd#GoodwillAndIntangibleAssetsDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/GoodwillAndIntangibleAssetsTables" xlink:href="dgx-20150930.xsd#GoodwillAndIntangibleAssetsTables" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/RelatedParties" xlink:href="dgx-20150930.xsd#RelatedParties" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/RelatedPartiesDetails" xlink:href="dgx-20150930.xsd#RelatedPartiesDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/RestructuringActivities" xlink:href="dgx-20150930.xsd#RestructuringActivities" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/RestructuringActivitiesActivitiesOfRestructuringLiabilitiesDetails" xlink:href="dgx-20150930.xsd#RestructuringActivitiesActivitiesOfRestructuringLiabilitiesDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/RestructuringActivitiesNarrativeDetails" xlink:href="dgx-20150930.xsd#RestructuringActivitiesNarrativeDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/RestructuringActivitiesPreTaxRestructuringAndIntegrationChargesDetails" xlink:href="dgx-20150930.xsd#RestructuringActivitiesPreTaxRestructuringAndIntegrationChargesDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/RestructuringActivitiesTables" xlink:href="dgx-20150930.xsd#RestructuringActivitiesTables" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/StockholdersEquityAndRedeemableNoncontrollingInterest" xlink:href="dgx-20150930.xsd#StockholdersEquityAndRedeemableNoncontrollingInterest" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/StockholdersEquityAndRedeemableNoncontrollingInterestNarrativeDetails" xlink:href="dgx-20150930.xsd#StockholdersEquityAndRedeemableNoncontrollingInterestNarrativeDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/SummaryOfSignificantAccountingPolicies" xlink:href="dgx-20150930.xsd#SummaryOfSignificantAccountingPolicies" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/SummaryOfSignificantAccountingPoliciesDetails" xlink:href="dgx-20150930.xsd#SummaryOfSignificantAccountingPoliciesDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/SummaryOfSignificantAccountingPoliciesPolicies" xlink:href="dgx-20150930.xsd#SummaryOfSignificantAccountingPoliciesPolicies" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/SupplementalCashFlowOtherData" xlink:href="dgx-20150930.xsd#SupplementalCashFlowOtherData" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/SupplementalCashFlowOtherDataDetails" xlink:href="dgx-20150930.xsd#SupplementalCashFlowOtherDataDetails" xlink:type="simple" />
  <roleRef roleURI="http://questdiagnostics.com/role/SupplementalCashFlowOtherDataTables" xlink:href="dgx-20150930.xsd#SupplementalCashFlowOtherDataTables" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" xlink:type="simple" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/BusinessAcquisition" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/BusinessAcquisitionNarrativeDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_EC2E6ECFD57A9337221F9052300895E7" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_7611773EEC5128448D1990523007250B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_EC2E6ECFD57A9337221F9052300895E7" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_7611773EEC5128448D1990523007250B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_9A28FBBF527705BDFBC79052300753ED" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_7611773EEC5128448D1990523007250B" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_9A28FBBF527705BDFBC79052300753ED" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9987F4626071FDF892ED90523008ADB2_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_9A28FBBF527705BDFBC79052300753ED" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9987F4626071FDF892ED90523008ADB2_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9987F4626071FDF892ED90523008ADB2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_9A28FBBF527705BDFBC79052300753ED" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9987F4626071FDF892ED90523008ADB2" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_MemorialCareMember" xlink:label="loc_dgx_MemorialCareMember_C6372B42E7D6F8472F8590523008FAC8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9987F4626071FDF892ED90523008ADB2" xlink:to="loc_dgx_MemorialCareMember_C6372B42E7D6F8472F8590523008FAC8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_112E6652A3F074F4CFE6905230082B1B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_EC2E6ECFD57A9337221F9052300895E7" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_112E6652A3F074F4CFE6905230082B1B" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/BusinessSegmentInformation" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/BusinessSegmentInformationDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_FC52DA1AD39F4D9745EBAE8452CCB510" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_4CBD48633DEC516E8809AE8452CBAEA0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_FC52DA1AD39F4D9745EBAE8452CCB510" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_4CBD48633DEC516E8809AE8452CBAEA0" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_5C54A61806435FAE4938AE8452CBFBF3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_4CBD48633DEC516E8809AE8452CBAEA0" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_5C54A61806435FAE4938AE8452CBFBF3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_47ED4B0D05D7B68AA548AE8452CC9DF9_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_5C54A61806435FAE4938AE8452CBFBF3" xlink:to="loc_us-gaap_SegmentDomain_47ED4B0D05D7B68AA548AE8452CC9DF9_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_47ED4B0D05D7B68AA548AE8452CC9DF9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_5C54A61806435FAE4938AE8452CBFBF3" xlink:to="loc_us-gaap_SegmentDomain_47ED4B0D05D7B68AA548AE8452CC9DF9" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_ClinicalLaboratoryTestingBusinessMember" xlink:label="loc_dgx_ClinicalLaboratoryTestingBusinessMember_3733281CF34579B9D949AE8452CC7EB8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_47ED4B0D05D7B68AA548AE8452CC9DF9" xlink:to="loc_dgx_ClinicalLaboratoryTestingBusinessMember_3733281CF34579B9D949AE8452CC7EB8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AllOtherSegmentsMember" xlink:label="loc_us-gaap_AllOtherSegmentsMember_995DCB1DFA832DC874B8AE8452CCCD2C" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_47ED4B0D05D7B68AA548AE8452CC9DF9" xlink:to="loc_us-gaap_AllOtherSegmentsMember_995DCB1DFA832DC874B8AE8452CCCD2C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CorporateMember" xlink:label="loc_us-gaap_CorporateMember_E15B4164506D7BE0543AAE8452CCE185" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_47ED4B0D05D7B68AA548AE8452CC9DF9" xlink:to="loc_us-gaap_CorporateMember_E15B4164506D7BE0543AAE8452CCE185" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_PercentageofNetRevenuesfromDiagnosticInformationServicesBusiness" xlink:label="loc_dgx_PercentageofNetRevenuesfromDiagnosticInformationServicesBusiness_FE73457AD1304CF06DCCAEAE99168151" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_FC52DA1AD39F4D9745EBAE8452CCB510" xlink:to="loc_dgx_PercentageofNetRevenuesfromDiagnosticInformationServicesBusiness_FE73457AD1304CF06DCCAEAE99168151" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueNet" xlink:label="loc_us-gaap_SalesRevenueNet_9B11A01FCD1EF0C72A93AE8452CC5391" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_FC52DA1AD39F4D9745EBAE8452CCB510" xlink:to="loc_us-gaap_SalesRevenueNet_9B11A01FCD1EF0C72A93AE8452CC5391" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OperatingIncomeLoss" xlink:label="loc_us-gaap_OperatingIncomeLoss_321E0186AA1D888F6FF6AE8452CC200D" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_FC52DA1AD39F4D9745EBAE8452CCB510" xlink:to="loc_us-gaap_OperatingIncomeLoss_321E0186AA1D888F6FF6AE8452CC200D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NonoperatingIncomeExpense" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_9CCA7B78AA66E8FBC8F3AE8452CCE7E8" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_FC52DA1AD39F4D9745EBAE8452CCB510" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_9CCA7B78AA66E8FBC8F3AE8452CCE7E8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments_5646A43C93967C1372FFAEB15189FFBE" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_FC52DA1AD39F4D9745EBAE8452CCB510" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments_5646A43C93967C1372FFAEB15189FFBE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_46B8A23135F2F62928CEAE8452CC6529" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_FC52DA1AD39F4D9745EBAE8452CCB510" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_46B8A23135F2F62928CEAE8452CC6529" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeLossFromEquityMethodInvestments" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_C49F1C586DE4772D027FAE8452CCC946" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_FC52DA1AD39F4D9745EBAE8452CCB510" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_C49F1C586DE4772D027FAE8452CCC946" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProfitLoss" xlink:label="loc_us-gaap_ProfitLoss_D47724D4B8E8351CA597AE8452CC89B9" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_FC52DA1AD39F4D9745EBAE8452CCB510" xlink:to="loc_us-gaap_ProfitLoss_D47724D4B8E8351CA597AE8452CC89B9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" xlink:label="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_16B969277A16184FD82AAE8452CDAE98" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_FC52DA1AD39F4D9745EBAE8452CCB510" xlink:to="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_16B969277A16184FD82AAE8452CDAE98" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_86F7DFA133DC210D888CAE8452CD68CF" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_FC52DA1AD39F4D9745EBAE8452CCB510" xlink:to="loc_us-gaap_NetIncomeLoss_86F7DFA133DC210D888CAE8452CD68CF" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/BusinessSegmentInformationTables" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/CommitmentsAndContingencies" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/CommitmentsAndContingenciesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongTermPurchaseCommitmentLineItems" xlink:label="loc_us-gaap_LongTermPurchaseCommitmentLineItems_506916180A5A2ABC356728FD57963743" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongTermPurchaseCommitmentTable" xlink:label="loc_us-gaap_LongTermPurchaseCommitmentTable_A2F16600A800F2CF4B2728FD5796F5A9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentLineItems_506916180A5A2ABC356728FD57963743" xlink:to="loc_us-gaap_LongTermPurchaseCommitmentTable_A2F16600A800F2CF4B2728FD5796F5A9" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_38136E718B0F080B8D5928FD5796AEBC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentTable_A2F16600A800F2CF4B2728FD5796F5A9" xlink:to="loc_us-gaap_CreditFacilityAxis_38136E718B0F080B8D5928FD5796AEBC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_156448447BF988BC2EF328FD5796132A_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_38136E718B0F080B8D5928FD5796AEBC" xlink:to="loc_us-gaap_CreditFacilityDomain_156448447BF988BC2EF328FD5796132A_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_156448447BF988BC2EF328FD5796132A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_38136E718B0F080B8D5928FD5796AEBC" xlink:to="loc_us-gaap_CreditFacilityDomain_156448447BF988BC2EF328FD5796132A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LineOfCreditMember" xlink:label="loc_us-gaap_LineOfCreditMember_2842C4E3EE999443017F28FD57967607" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_156448447BF988BC2EF328FD5796132A" xlink:to="loc_us-gaap_LineOfCreditMember_2842C4E3EE999443017F28FD57967607" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity" xlink:label="loc_us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity_800345D11F28B4BBE92528FD5796BDE5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentLineItems_506916180A5A2ABC356728FD57963743" xlink:to="loc_us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity_800345D11F28B4BBE92528FD5796BDE5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentMaturityDate" xlink:label="loc_us-gaap_DebtInstrumentMaturityDate_B534864DF2EAB4BAA12A28FD5796C3E2" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentLineItems_506916180A5A2ABC356728FD57963743" xlink:to="loc_us-gaap_DebtInstrumentMaturityDate_B534864DF2EAB4BAA12A28FD5796C3E2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LettersOfCreditOutstandingAmount" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_0FA8550D4CA42DADEB7928FD57967A34" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentLineItems_506916180A5A2ABC356728FD57963743" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_0FA8550D4CA42DADEB7928FD57967A34" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LitigationReserve" xlink:label="loc_us-gaap_LitigationReserve_DA6105697C8EF06B364028FD57967577" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentLineItems_506916180A5A2ABC356728FD57963743" xlink:to="loc_us-gaap_LitigationReserve_DA6105697C8EF06B364028FD57967577" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MalpracticeLossContingencyAccrualUndiscounted" xlink:label="loc_us-gaap_MalpracticeLossContingencyAccrualUndiscounted_74FAC62D9DD1DAAAB29828FD5796C834" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentLineItems_506916180A5A2ABC356728FD57963743" xlink:to="loc_us-gaap_MalpracticeLossContingencyAccrualUndiscounted_74FAC62D9DD1DAAAB29828FD5796C834" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/ConsolidatedBalanceSheets" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/ConsolidatedBalanceSheetsParenthetical" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/ConsolidatedStatementsOfCashFlows" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/ConsolidatedStatementsOfComprehensiveIncome" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/ConsolidatedStatementsOfOperations" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/ConsolidatedStatementsOfStockholdersEquity" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_16AFE677056457A053DCAF29C3B64F90" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_995A7CEA5514A0E3D7BAAF29C3B03686" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_16AFE677056457A053DCAF29C3B64F90" xlink:to="loc_us-gaap_StatementTable_995A7CEA5514A0E3D7BAAF29C3B03686" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_9A9BCFB8495A584AA33DAF29C3B0958A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_995A7CEA5514A0E3D7BAAF29C3B03686" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_9A9BCFB8495A584AA33DAF29C3B0958A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_366093734C0073DF340AAF29C3B05085_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_9A9BCFB8495A584AA33DAF29C3B0958A" xlink:to="loc_us-gaap_EquityComponentDomain_366093734C0073DF340AAF29C3B05085_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_366093734C0073DF340AAF29C3B05085" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_9A9BCFB8495A584AA33DAF29C3B0958A" xlink:to="loc_us-gaap_EquityComponentDomain_366093734C0073DF340AAF29C3B05085" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_CB65716016C4B95B051DAF29C3B182A7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_366093734C0073DF340AAF29C3B05085" xlink:to="loc_us-gaap_CommonStockMember_CB65716016C4B95B051DAF29C3B182A7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_B19899AD001B52F87DF1AF29C3B1F6E9" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_366093734C0073DF340AAF29C3B05085" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_B19899AD001B52F87DF1AF29C3B1F6E9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="loc_us-gaap_RetainedEarningsMember_7927DBCB46BB28E8650DAF29C3B1C083" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_366093734C0073DF340AAF29C3B05085" xlink:to="loc_us-gaap_RetainedEarningsMember_7927DBCB46BB28E8650DAF29C3B1C083" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_C96C3312D599B6E6741FAF29C3B10A50" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_366093734C0073DF340AAF29C3B05085" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_C96C3312D599B6E6741FAF29C3B10A50" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TreasuryStockMember" xlink:label="loc_us-gaap_TreasuryStockMember_FD71FF2B9E5E6FB79A35AF29C3B6DB99" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_366093734C0073DF340AAF29C3B05085" xlink:to="loc_us-gaap_TreasuryStockMember_FD71FF2B9E5E6FB79A35AF29C3B6DB99" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NoncontrollingInterestMember" xlink:label="loc_us-gaap_NoncontrollingInterestMember_DAB35BE0F8F9C82F84EDAF29C3B61DA9" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_366093734C0073DF340AAF29C3B05085" xlink:to="loc_us-gaap_NoncontrollingInterestMember_DAB35BE0F8F9C82F84EDAF29C3B61DA9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_16AFE677056457A053DCAF29C3B64F90" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_6DB76683CC79EF713C50AF29C3B6C481" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_6DB76683CC79EF713C50AF29C3B6C481" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_01E1941E2B271FD1784AAF29C3B72415" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_01E1941E2B271FD1784AAF29C3B72415" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_NetIncomeLossIncludingPortionAttributabletoNoncontrollingInterestExcludingRedeemableNoncontrollingInterest" xlink:label="loc_dgx_NetIncomeLossIncludingPortionAttributabletoNoncontrollingInterestExcludingRedeemableNoncontrollingInterest_77CE72D2FDAE74D44A41AF29C3B75EA9" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_dgx_NetIncomeLossIncludingPortionAttributabletoNoncontrollingInterestExcludingRedeemableNoncontrollingInterest_77CE72D2FDAE74D44A41AF29C3B75EA9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProfitLoss" xlink:label="loc_us-gaap_ProfitLoss_FB1894FCC176231B2735AF29C3B7FDCF" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_us-gaap_ProfitLoss_FB1894FCC176231B2735AF29C3B7FDCF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_84A1F4B1AEC107988520AF29C3B7FD6F" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_84A1F4B1AEC107988520AF29C3B7FD6F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DividendsCommonStockCash" xlink:label="loc_us-gaap_DividendsCommonStockCash_103446929251BA2ABB35AF29C3B7B7B0" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_us-gaap_DividendsCommonStockCash_103446929251BA2ABB35AF29C3B7B7B0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders" xlink:label="loc_us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders_EA5F920D37B91FE5BB06AF29C3B7E8BA" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders_EA5F920D37B91FE5BB06AF29C3B7E8BA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_DEA410F47CC8632B2DF6AF29C3B8BE68" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_DEA410F47CC8632B2DF6AF29C3B8BE68" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_5BF1E975DF8A68E5559DAF29C3B87C8D" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_5BF1E975DF8A68E5559DAF29C3B87C8D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_840AD1501E605B4A60BDAF29C3B8E270" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_840AD1501E605B4A60BDAF29C3B8E270" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_09B97A0D56A1668EFBCEAF29C3B86D74" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_09B97A0D56A1668EFBCEAF29C3B86D74" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_C2664787C4262B33AEF9AF29C3B864C5" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_C2664787C4262B33AEF9AF29C3B864C5" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_Sharestocoveremployeepayrolltaxwithholdingsonstockbenefitplansvalue" xlink:label="loc_dgx_Sharestocoveremployeepayrolltaxwithholdingsonstockbenefitplansvalue_3EE3746F26C85D64B80AAF29C3B8D490" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_dgx_Sharestocoveremployeepayrolltaxwithholdingsonstockbenefitplansvalue_3EE3746F26C85D64B80AAF29C3B8D490" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation_59C6005C7BDB9BFF3480AF29C3B872BF" xlink:type="locator" />
    <definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation_59C6005C7BDB9BFF3480AF29C3B872BF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_69CBBAA2E09EBF69CEA5AF29C3B8AAB7" xlink:type="locator" />
    <definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_69CBBAA2E09EBF69CEA5AF29C3B8AAB7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TreasuryStockSharesAcquired" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_6E016AF774243713C85CAF29C3B8490A" xlink:type="locator" />
    <definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_6E016AF774243713C85CAF29C3B8490A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NoncontrollingInterestIncreaseFromSaleOfParentEquityInterest" xlink:label="loc_us-gaap_NoncontrollingInterestIncreaseFromSaleOfParentEquityInterest_234AF7D452A19FC7195BAF29C3B8C3D8" xlink:type="locator" />
    <definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_us-gaap_NoncontrollingInterestIncreaseFromSaleOfParentEquityInterest_234AF7D452A19FC7195BAF29C3B8C3D8" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_TemporaryEquityAdjustmentToRedemptionValue" xlink:label="loc_dgx_TemporaryEquityAdjustmentToRedemptionValue_525B2DCC291C6AB11FC5AF29C3B82EAF" xlink:type="locator" />
    <definitionArc order="18" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E6FE9CF61980D66BB9AAAF29C3B6B75E" xlink:to="loc_dgx_TemporaryEquityAdjustmentToRedemptionValue_525B2DCC291C6AB11FC5AF29C3B82EAF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncreaseDecreaseInTemporaryEquityRollForward" xlink:label="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_91B1F60EFBCC23AAE890AF29C3B97DB9" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_16AFE677056457A053DCAF29C3B64F90" xlink:to="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_91B1F60EFBCC23AAE890AF29C3B97DB9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TemporaryEquityCarryingAmountIncludingPortionAttributableToNoncontrollingInterests" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountIncludingPortionAttributableToNoncontrollingInterests_9A459AE620C5303698A1AF29C3B96673" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_91B1F60EFBCC23AAE890AF29C3B97DB9" xlink:to="loc_us-gaap_TemporaryEquityCarryingAmountIncludingPortionAttributableToNoncontrollingInterests_9A459AE620C5303698A1AF29C3B96673" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TemporaryEquityNetIncome" xlink:label="loc_us-gaap_TemporaryEquityNetIncome_0C459DA07BA30C031C93AF29C3B9B252" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_91B1F60EFBCC23AAE890AF29C3B97DB9" xlink:to="loc_us-gaap_TemporaryEquityNetIncome_0C459DA07BA30C031C93AF29C3B9B252" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues" xlink:label="loc_us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues_7F89D34BBC701B16DEE3AF29C3B95BB1" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_91B1F60EFBCC23AAE890AF29C3B97DB9" xlink:to="loc_us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues_7F89D34BBC701B16DEE3AF29C3B95BB1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TemporaryEquityAccretionToRedemptionValue" xlink:label="loc_us-gaap_TemporaryEquityAccretionToRedemptionValue_D73033AAE72A61B3D4BBAF29C3B9C7B4" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_91B1F60EFBCC23AAE890AF29C3B97DB9" xlink:to="loc_us-gaap_TemporaryEquityAccretionToRedemptionValue_D73033AAE72A61B3D4BBAF29C3B9C7B4" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/Debt" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/DebtLongTermDebtDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_3FBDCB55D564C9143E7EAF31C7E99AC5" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:label="loc_us-gaap_DebtInstrumentTable_1800126E740E62F039A8AF31C7E7CF1E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_3FBDCB55D564C9143E7EAF31C7E99AC5" xlink:to="loc_us-gaap_DebtInstrumentTable_1800126E740E62F039A8AF31C7E7CF1E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_02C990BFFE8D848AEE55AF31C7E7FEE9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_1800126E740E62F039A8AF31C7E7CF1E" xlink:to="loc_us-gaap_CreditFacilityAxis_02C990BFFE8D848AEE55AF31C7E7FEE9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_CA1621E0E9983C45DDDFAF31C7E78BE2_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_02C990BFFE8D848AEE55AF31C7E7FEE9" xlink:to="loc_us-gaap_CreditFacilityDomain_CA1621E0E9983C45DDDFAF31C7E78BE2_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_CA1621E0E9983C45DDDFAF31C7E78BE2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_02C990BFFE8D848AEE55AF31C7E7FEE9" xlink:to="loc_us-gaap_CreditFacilityDomain_CA1621E0E9983C45DDDFAF31C7E78BE2" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_SecuredReceivablesCreditFacilityMember" xlink:label="loc_dgx_SecuredReceivablesCreditFacilityMember_C25CC09B5C667F3FA057AF31C7E76BAA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_CA1621E0E9983C45DDDFAF31C7E78BE2" xlink:to="loc_dgx_SecuredReceivablesCreditFacilityMember_C25CC09B5C667F3FA057AF31C7E76BAA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_FC27430CBD8C6D41AD50AF31C7E746FE" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_1800126E740E62F039A8AF31C7E7CF1E" xlink:to="loc_us-gaap_DebtInstrumentAxis_FC27430CBD8C6D41AD50AF31C7E746FE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_FC27430CBD8C6D41AD50AF31C7E746FE" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_FC27430CBD8C6D41AD50AF31C7E746FE" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_FivePointFourFivePercentSeniorNotesDue2015Member" xlink:label="loc_dgx_FivePointFourFivePercentSeniorNotesDue2015Member_92493B2EE75709D05EEDAF31C7E86C29" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14" xlink:to="loc_dgx_FivePointFourFivePercentSeniorNotesDue2015Member_92493B2EE75709D05EEDAF31C7E86C29" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_ThreePointTwoZeroPercentSeniorNotesDue2016Member" xlink:label="loc_dgx_ThreePointTwoZeroPercentSeniorNotesDue2016Member_B66BADF845E79F5A3866AF31C7E83DB7" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14" xlink:to="loc_dgx_ThreePointTwoZeroPercentSeniorNotesDue2016Member_B66BADF845E79F5A3866AF31C7E83DB7" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_SixPointFourPercentSeniorNotesDue2017Member" xlink:label="loc_dgx_SixPointFourPercentSeniorNotesDue2017Member_8354CE0AE670089AD16EAF31C7E8DCC3" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14" xlink:to="loc_dgx_SixPointFourPercentSeniorNotesDue2017Member_8354CE0AE670089AD16EAF31C7E8DCC3" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_TwoPointSevenPercentSeniorNotesDue2019Member" xlink:label="loc_dgx_TwoPointSevenPercentSeniorNotesDue2019Member_382734384100FD9D2EFAAF31C7E8450A" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14" xlink:to="loc_dgx_TwoPointSevenPercentSeniorNotesDue2019Member_382734384100FD9D2EFAAF31C7E8450A" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_FourPointSevenFivePercentSeniorNotesDue2020Member" xlink:label="loc_dgx_FourPointSevenFivePercentSeniorNotesDue2020Member_CB473027A4E3510760E4AF31C7E8A1D6" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14" xlink:to="loc_dgx_FourPointSevenFivePercentSeniorNotesDue2020Member_CB473027A4E3510760E4AF31C7E8A1D6" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_TwoPointFiveZeroPercentSeniorNotesdueMarch2020Member" xlink:label="loc_dgx_TwoPointFiveZeroPercentSeniorNotesdueMarch2020Member_5C266FB7AD541AE45689AF31C7E81726" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14" xlink:to="loc_dgx_TwoPointFiveZeroPercentSeniorNotesdueMarch2020Member_5C266FB7AD541AE45689AF31C7E81726" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_FourPointSevenZeroPercentSeniorNotesdue2021Member" xlink:label="loc_dgx_FourPointSevenZeroPercentSeniorNotesdue2021Member_0B39909DDB25A58E402FAF31C7E8A144" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14" xlink:to="loc_dgx_FourPointSevenZeroPercentSeniorNotesdue2021Member_0B39909DDB25A58E402FAF31C7E8A144" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_FourpointtwofivepercentSeniorNotesDue2024Member" xlink:label="loc_dgx_FourpointtwofivepercentSeniorNotesDue2024Member_8B76B43A42CEFF8BA452AF31C7E83C1C" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14" xlink:to="loc_dgx_FourpointtwofivepercentSeniorNotesDue2024Member_8B76B43A42CEFF8BA452AF31C7E83C1C" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_ThreePointFiveZeroPercentSeniorNotesdueMarch2025Member" xlink:label="loc_dgx_ThreePointFiveZeroPercentSeniorNotesdueMarch2025Member_1AA861D230331AE67C08AF31C7E84DDD" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14" xlink:to="loc_dgx_ThreePointFiveZeroPercentSeniorNotesdueMarch2025Member_1AA861D230331AE67C08AF31C7E84DDD" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_SixPointNineFivePercentSeniorNotesDue2037Member" xlink:label="loc_dgx_SixPointNineFivePercentSeniorNotesDue2037Member_AB4EA62B96C8AC51D442AF31C7E88211" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14" xlink:to="loc_dgx_SixPointNineFivePercentSeniorNotesDue2037Member_AB4EA62B96C8AC51D442AF31C7E88211" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_FivePointSevenFivePercentSeniorNotesDue2040Member" xlink:label="loc_dgx_FivePointSevenFivePercentSeniorNotesDue2040Member_722C373497C770833E15AF31C7E8D47A" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14" xlink:to="loc_dgx_FivePointSevenFivePercentSeniorNotesDue2040Member_722C373497C770833E15AF31C7E8D47A" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_FourPointSevenZeroPercentSeniorNotesdueMarch2045Member" xlink:label="loc_dgx_FourPointSevenZeroPercentSeniorNotesdueMarch2045Member_9544F9EA982500441189AF31C7E82E8E" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14" xlink:to="loc_dgx_FourPointSevenZeroPercentSeniorNotesdueMarch2045Member_9544F9EA982500441189AF31C7E82E8E" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_OtherLongTermDebtMember" xlink:label="loc_dgx_OtherLongTermDebtMember_E7A9C06CE7BF5266F901AF31C7E873FA" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_952E80AC5F84E78F2C92AF31C7E74F14" xlink:to="loc_dgx_OtherLongTermDebtMember_E7A9C06CE7BF5266F901AF31C7E873FA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_D4E708EE40EC88B0E488AF31C7E81526" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_1800126E740E62F039A8AF31C7E7CF1E" xlink:to="loc_us-gaap_RangeAxis_D4E708EE40EC88B0E488AF31C7E81526" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_D5185B2174EE50D640D0AF31C7E91082_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_D4E708EE40EC88B0E488AF31C7E81526" xlink:to="loc_us-gaap_RangeMember_D5185B2174EE50D640D0AF31C7E91082_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_D5185B2174EE50D640D0AF31C7E91082" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_D4E708EE40EC88B0E488AF31C7E81526" xlink:to="loc_us-gaap_RangeMember_D5185B2174EE50D640D0AF31C7E91082" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_955BF77A9984AB965745AF31C7E966CB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_D5185B2174EE50D640D0AF31C7E91082" xlink:to="loc_us-gaap_MinimumMember_955BF77A9984AB965745AF31C7E966CB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_039113B0842CAF98064EAF31C7E985B5" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_D5185B2174EE50D640D0AF31C7E91082" xlink:to="loc_us-gaap_MaximumMember_039113B0842CAF98064EAF31C7E985B5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongTermDebt" xlink:label="loc_us-gaap_LongTermDebt_4EA27ABA50288649288AAF31C7E97B2C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_3FBDCB55D564C9143E7EAF31C7E99AC5" xlink:to="loc_us-gaap_LongTermDebt_4EA27ABA50288649288AAF31C7E97B2C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherLongTermDebt" xlink:label="loc_us-gaap_OtherLongTermDebt_D68B3DA4871BF99DF4D3AF31C7E9BD54" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_3FBDCB55D564C9143E7EAF31C7E99AC5" xlink:to="loc_us-gaap_OtherLongTermDebt_D68B3DA4871BF99DF4D3AF31C7E9BD54" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_UnamortizedDebtIssuanceExpense" xlink:label="loc_us-gaap_UnamortizedDebtIssuanceExpense_4899ED688BC0E333BED3AF31C7E98988" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_3FBDCB55D564C9143E7EAF31C7E99AC5" xlink:to="loc_us-gaap_UnamortizedDebtIssuanceExpense_4899ED688BC0E333BED3AF31C7E98988" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongTermDebtCurrent" xlink:label="loc_us-gaap_LongTermDebtCurrent_32226DF48173CFEFCB06AF31C7E9B472" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_3FBDCB55D564C9143E7EAF31C7E99AC5" xlink:to="loc_us-gaap_LongTermDebtCurrent_32226DF48173CFEFCB06AF31C7E9B472" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongTermDebtNoncurrent" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_DF8DBEB3B293BE18AC05AF31C7E9CB65" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_3FBDCB55D564C9143E7EAF31C7E99AC5" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_DF8DBEB3B293BE18AC05AF31C7E9CB65" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_0CBD33421BFE247FA440AF31C7E97EB7" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_3FBDCB55D564C9143E7EAF31C7E99AC5" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_0CBD33421BFE247FA440AF31C7E97EB7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentMaturityDate" xlink:label="loc_us-gaap_DebtInstrumentMaturityDate_DC069A3089BF6C261DF0AF31C7E9E16E" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_3FBDCB55D564C9143E7EAF31C7E99AC5" xlink:to="loc_us-gaap_DebtInstrumentMaturityDate_DC069A3089BF6C261DF0AF31C7E9E16E" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/DebtMaturitiesOfLongTermDebtDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/DebtNarrativeDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_53FBF69E2B1C26BD5C56954B4CA11175" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:label="loc_us-gaap_DebtInstrumentTable_0C86E0D242B228F0A66A954B4C9A79B3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_53FBF69E2B1C26BD5C56954B4CA11175" xlink:to="loc_us-gaap_DebtInstrumentTable_0C86E0D242B228F0A66A954B4C9A79B3" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_8B5E4C3753015FEBD5C8954B4C9A5212" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_0C86E0D242B228F0A66A954B4C9A79B3" xlink:to="loc_us-gaap_DebtInstrumentAxis_8B5E4C3753015FEBD5C8954B4C9A5212" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_B0B3151E4D2346C69817954B4C9AE6E1_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_8B5E4C3753015FEBD5C8954B4C9A5212" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_B0B3151E4D2346C69817954B4C9AE6E1_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_B0B3151E4D2346C69817954B4C9AE6E1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_8B5E4C3753015FEBD5C8954B4C9A5212" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_B0B3151E4D2346C69817954B4C9AE6E1" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_TwentyFifteenSeniorNotesMember" xlink:label="loc_dgx_TwentyFifteenSeniorNotesMember_EDBC5972C2B93C65939C954B4C9BD81E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_B0B3151E4D2346C69817954B4C9AE6E1" xlink:to="loc_dgx_TwentyFifteenSeniorNotesMember_EDBC5972C2B93C65939C954B4C9BD81E" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_TwoPointFiveZeroPercentSeniorNotesdueMarch2020Member" xlink:label="loc_dgx_TwoPointFiveZeroPercentSeniorNotesdueMarch2020Member_F4EAAC83FE6896505506954B4C9B5482" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_B0B3151E4D2346C69817954B4C9AE6E1" xlink:to="loc_dgx_TwoPointFiveZeroPercentSeniorNotesdueMarch2020Member_F4EAAC83FE6896505506954B4C9B5482" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_ThreePointFiveZeroPercentSeniorNotesdueMarch2025Member" xlink:label="loc_dgx_ThreePointFiveZeroPercentSeniorNotesdueMarch2025Member_B0AE0B110402C37593BF954B4C9B61CF" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_B0B3151E4D2346C69817954B4C9AE6E1" xlink:to="loc_dgx_ThreePointFiveZeroPercentSeniorNotesdueMarch2025Member_B0AE0B110402C37593BF954B4C9B61CF" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_FourPointSevenZeroPercentSeniorNotesdueMarch2045Member" xlink:label="loc_dgx_FourPointSevenZeroPercentSeniorNotesdueMarch2045Member_C73A634CC3B9D373207E954B4C9B859C" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_B0B3151E4D2346C69817954B4C9AE6E1" xlink:to="loc_dgx_FourPointSevenZeroPercentSeniorNotesdueMarch2045Member_C73A634CC3B9D373207E954B4C9B859C" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_SixpointNineFivePercentSeniorNotesdueJuly2037and5.75SeniorNotesdueJanuary2040Member" xlink:label="loc_dgx_SixpointNineFivePercentSeniorNotesdueJuly2037and5.75SeniorNotesdueJanuary2040Member_50A2D6F4E17767C9864F954B4C9B956B" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_B0B3151E4D2346C69817954B4C9AE6E1" xlink:to="loc_dgx_SixpointNineFivePercentSeniorNotesdueJuly2037and5.75SeniorNotesdueJanuary2040Member_50A2D6F4E17767C9864F954B4C9B956B" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_SixPointNineFivePercentSeniorNotesDue2037Member" xlink:label="loc_dgx_SixPointNineFivePercentSeniorNotesDue2037Member_FFD0AF29E2E489D77F50954B4C9B0EBE" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_B0B3151E4D2346C69817954B4C9AE6E1" xlink:to="loc_dgx_SixPointNineFivePercentSeniorNotesDue2037Member_FFD0AF29E2E489D77F50954B4C9B0EBE" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_FivePointSevenFivePercentSeniorNotesDue2040Member" xlink:label="loc_dgx_FivePointSevenFivePercentSeniorNotesDue2040Member_DD646FFE54B661E3566B954B4C9BAB81" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_B0B3151E4D2346C69817954B4C9AE6E1" xlink:to="loc_dgx_FivePointSevenFivePercentSeniorNotesDue2040Member_DD646FFE54B661E3566B954B4C9BAB81" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_FivePointFourFivePercentSeniorNotesDue2015Member" xlink:label="loc_dgx_FivePointFourFivePercentSeniorNotesDue2015Member_EF34C7B150DA06FD4F13954B4C9B6FD3" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_B0B3151E4D2346C69817954B4C9AE6E1" xlink:to="loc_dgx_FivePointFourFivePercentSeniorNotesDue2015Member_EF34C7B150DA06FD4F13954B4C9B6FD3" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_ThreePointTwoZeroPercentSeniorNotesDue2016Member" xlink:label="loc_dgx_ThreePointTwoZeroPercentSeniorNotesDue2016Member_4EB472E3BC95AD257276954B4C9B6451" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_B0B3151E4D2346C69817954B4C9AE6E1" xlink:to="loc_dgx_ThreePointTwoZeroPercentSeniorNotesDue2016Member_4EB472E3BC95AD257276954B4C9B6451" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_SixPointFourPercentSeniorNotesDue2017Member" xlink:label="loc_dgx_SixPointFourPercentSeniorNotesDue2017Member_656E19590F3DDEE9952C954B4CA0D5A5" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_B0B3151E4D2346C69817954B4C9AE6E1" xlink:to="loc_dgx_SixPointFourPercentSeniorNotesDue2017Member_656E19590F3DDEE9952C954B4CA0D5A5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_BD01638D0366208A8609954B4CA00959" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_0C86E0D242B228F0A66A954B4C9A79B3" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_BD01638D0366208A8609954B4CA00959" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_97EDE594BFFE8C88B3F7954B4CA0E2CA_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_BD01638D0366208A8609954B4CA00959" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_97EDE594BFFE8C88B3F7954B4CA0E2CA_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_97EDE594BFFE8C88B3F7954B4CA0E2CA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_BD01638D0366208A8609954B4CA00959" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_97EDE594BFFE8C88B3F7954B4CA0E2CA" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_OtherIncomeExpenseNetMember" xlink:label="loc_dgx_OtherIncomeExpenseNetMember_37ACE79B78E59FC36063954B4CA06469" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_97EDE594BFFE8C88B3F7954B4CA0E2CA" xlink:to="loc_dgx_OtherIncomeExpenseNetMember_37ACE79B78E59FC36063954B4CA06469" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentFaceAmount" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_69AD1D198E1E3D02FBCA954B4CA1A65D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_53FBF69E2B1C26BD5C56954B4CA11175" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_69AD1D198E1E3D02FBCA954B4CA1A65D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_4143D742622240BB09CA954B4CA1DA60" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_53FBF69E2B1C26BD5C56954B4CA11175" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_4143D742622240BB09CA954B4CA1DA60" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentUnamortizedDiscount" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_84334340F112E0F16F22954B4CA11FD5" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_53FBF69E2B1C26BD5C56954B4CA11175" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_84334340F112E0F16F22954B4CA11FD5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtIssuanceCosts" xlink:label="loc_us-gaap_DebtIssuanceCosts_5F019CCC47A5CDB4FD33954B4CA19C02" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_53FBF69E2B1C26BD5C56954B4CA11175" xlink:to="loc_us-gaap_DebtIssuanceCosts_5F019CCC47A5CDB4FD33954B4CA19C02" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ExtinguishmentOfDebtAmount" xlink:label="loc_us-gaap_ExtinguishmentOfDebtAmount_E04F6C04C3242DB4E718954B4CA1C25A" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_53FBF69E2B1C26BD5C56954B4CA11175" xlink:to="loc_us-gaap_ExtinguishmentOfDebtAmount_E04F6C04C3242DB4E718954B4CA1C25A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_AB40C70E3054F02300B9954B4CA11999" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_53FBF69E2B1C26BD5C56954B4CA11175" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_AB40C70E3054F02300B9954B4CA11999" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/DebtTables" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/DescriptionOfBusiness" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/DispositionAndHeldForSale" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/DispositionAndHeldForSaleDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_2E4B9E218BD4BE7946F3AF31C82B723B" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_CF013FC62DF339CA9178AF31C82BC44B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_2E4B9E218BD4BE7946F3AF31C82B723B" xlink:to="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_CF013FC62DF339CA9178AF31C82BC44B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_4ACF96E80987944331F6AF31C82B98BE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_CF013FC62DF339CA9178AF31C82BC44B" xlink:to="loc_dei_LegalEntityAxis_4ACF96E80987944331F6AF31C82B98BE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_5A12A373D30D6B3F336BAF31C82B8584_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_4ACF96E80987944331F6AF31C82B98BE" xlink:to="loc_dei_EntityDomain_5A12A373D30D6B3F336BAF31C82B8584_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_5A12A373D30D6B3F336BAF31C82B8584" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_4ACF96E80987944331F6AF31C82B98BE" xlink:to="loc_dei_EntityDomain_5A12A373D30D6B3F336BAF31C82B8584" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_QSolutionsMember" xlink:label="loc_dgx_QSolutionsMember_00B1BA3CF8CB3EB268F8AF31C82B8942" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_5A12A373D30D6B3F336BAF31C82B8584" xlink:to="loc_dgx_QSolutionsMember_00B1BA3CF8CB3EB268F8AF31C82B8942" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_ClinicalTrialsMember" xlink:label="loc_dgx_ClinicalTrialsMember_F3951072548909F0E7AEAF31C82BF1E0" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_5A12A373D30D6B3F336BAF31C82B8584" xlink:to="loc_dgx_ClinicalTrialsMember_F3951072548909F0E7AEAF31C82BF1E0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisposalGroupClassificationAxis" xlink:label="loc_us-gaap_DisposalGroupClassificationAxis_4B81399915DE2A48BA10AF31C82BE30B" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_CF013FC62DF339CA9178AF31C82BC44B" xlink:to="loc_us-gaap_DisposalGroupClassificationAxis_4B81399915DE2A48BA10AF31C82BE30B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisposalGroupClassificationDomain" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_EF2EB16587F7E61A9B74AF31C82B8562_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_4B81399915DE2A48BA10AF31C82BE30B" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_EF2EB16587F7E61A9B74AF31C82B8562_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisposalGroupClassificationDomain" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_EF2EB16587F7E61A9B74AF31C82B8562" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_4B81399915DE2A48BA10AF31C82BE30B" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_EF2EB16587F7E61A9B74AF31C82B8562" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_EquityMethodInvestmentBasisDifferenceMember" xlink:label="loc_dgx_EquityMethodInvestmentBasisDifferenceMember_CC6392DCEDFDB4820BABAF31C82B5EE0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_EF2EB16587F7E61A9B74AF31C82B8562" xlink:to="loc_dgx_EquityMethodInvestmentBasisDifferenceMember_CC6392DCEDFDB4820BABAF31C82B5EE0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PaymentsToAcquireEquityMethodInvestments" xlink:label="loc_us-gaap_PaymentsToAcquireEquityMethodInvestments_CC6477F1DF25F44A0CBEAF31C82B3AED" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_2E4B9E218BD4BE7946F3AF31C82B723B" xlink:to="loc_us-gaap_PaymentsToAcquireEquityMethodInvestments_CC6477F1DF25F44A0CBEAF31C82B3AED" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_9221F74AD18618DAD290AF31C82BE486" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_2E4B9E218BD4BE7946F3AF31C82B723B" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_9221F74AD18618DAD290AF31C82BE486" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityMethodInvestments" xlink:label="loc_us-gaap_EquityMethodInvestments_B398D0270A7B1692324AAF31C82BCC26" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_2E4B9E218BD4BE7946F3AF31C82B723B" xlink:to="loc_us-gaap_EquityMethodInvestments_B398D0270A7B1692324AAF31C82BCC26" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GainLossOnSaleOfBusiness" xlink:label="loc_us-gaap_GainLossOnSaleOfBusiness_966741DE3B38E95C518FAF31C82B43D3" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_2E4B9E218BD4BE7946F3AF31C82B723B" xlink:to="loc_us-gaap_GainLossOnSaleOfBusiness_966741DE3B38E95C518FAF31C82B43D3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredIncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_841249D591E0532D2B5EAF31C82B9019" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_2E4B9E218BD4BE7946F3AF31C82B723B" xlink:to="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_841249D591E0532D2B5EAF31C82B9019" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity" xlink:label="loc_us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity_E5DF5CA0D0495A030391AF31C82CFC0D" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_2E4B9E218BD4BE7946F3AF31C82B723B" xlink:to="loc_us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity_E5DF5CA0D0495A030391AF31C82CFC0D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsGross" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_9607452563EA923E9232AF31C82C95FF" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_2E4B9E218BD4BE7946F3AF31C82B723B" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_9607452563EA923E9232AF31C82C95FF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1_55DCD474A415168C43C5AF31C82CEED9" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_2E4B9E218BD4BE7946F3AF31C82B723B" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1_55DCD474A415168C43C5AF31C82CEED9" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_GoodwillReclassifiedtoCurrentAssetsHeldforsale" xlink:label="loc_dgx_GoodwillReclassifiedtoCurrentAssetsHeldforsale_00D5858CE63C02D3A37AAF3A04D8B995" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_2E4B9E218BD4BE7946F3AF31C82B723B" xlink:to="loc_dgx_GoodwillReclassifiedtoCurrentAssetsHeldforsale_00D5858CE63C02D3A37AAF3A04D8B995" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/DocumentAndEntityInformation" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/EarningsPerShare" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/EarningsPerShareDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/EarningsPerShareTables" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/FairValueMeasurements" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/FairValueMeasurementsNarrativeDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_20059A9DA172EC6159E1AF31C81C26E2" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_290D79FDAA2DFB437C7BAF31C81BAAE9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_20059A9DA172EC6159E1AF31C81C26E2" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_290D79FDAA2DFB437C7BAF31C81BAAE9" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_B8F32B555639A24198AFAF31C81B89E2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_290D79FDAA2DFB437C7BAF31C81BAAE9" xlink:to="loc_dei_LegalEntityAxis_B8F32B555639A24198AFAF31C81B89E2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_76234DAA6716605904E5AF31C81BB4D1_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_B8F32B555639A24198AFAF31C81B89E2" xlink:to="loc_dei_EntityDomain_76234DAA6716605904E5AF31C81BB4D1_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_76234DAA6716605904E5AF31C81BB4D1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_B8F32B555639A24198AFAF31C81B89E2" xlink:to="loc_dei_EntityDomain_76234DAA6716605904E5AF31C81BB4D1" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_UMassMember" xlink:label="loc_dgx_UMassMember_8E9AB8982352F4D7DC0CAF31C81B16CF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_76234DAA6716605904E5AF31C81BB4D1" xlink:to="loc_dgx_UMassMember_8E9AB8982352F4D7DC0CAF31C81B16CF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_12E1C319404B5814FE18AF31C81BE7BB" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_290D79FDAA2DFB437C7BAF31C81BAAE9" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_12E1C319404B5814FE18AF31C81BE7BB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_42002A9FD8963138558DAF31C81BE68B_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_12E1C319404B5814FE18AF31C81BE7BB" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_42002A9FD8963138558DAF31C81BE68B_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_42002A9FD8963138558DAF31C81BE68B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_12E1C319404B5814FE18AF31C81BE7BB" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_42002A9FD8963138558DAF31C81BE68B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SubsidiariesMember" xlink:label="loc_us-gaap_SubsidiariesMember_C4C36356FB4985303F6CAF31C81B5721" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_42002A9FD8963138558DAF31C81BE68B" xlink:to="loc_us-gaap_SubsidiariesMember_C4C36356FB4985303F6CAF31C81B5721" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_SummitHealthInc.Member" xlink:label="loc_dgx_SummitHealthInc.Member_50742B4B7E8ACDFE8DB9AF31C81BE0BA" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_42002A9FD8963138558DAF31C81BE68B" xlink:to="loc_dgx_SummitHealthInc.Member_50742B4B7E8ACDFE8DB9AF31C81BE0BA" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_StewardHealthCareSystemsLLCMemberMember" xlink:label="loc_dgx_StewardHealthCareSystemsLLCMemberMember_239F06B46BAF3501BAD7AF31C81B1768" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_42002A9FD8963138558DAF31C81BE68B" xlink:to="loc_dgx_StewardHealthCareSystemsLLCMemberMember_239F06B46BAF3501BAD7AF31C81B1768" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OptionIndexedToIssuersEquityTypeAxis" xlink:label="loc_us-gaap_OptionIndexedToIssuersEquityTypeAxis_A217D13FD08765FFA2B8AF31C81B0AA9" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_290D79FDAA2DFB437C7BAF31C81BAAE9" xlink:to="loc_us-gaap_OptionIndexedToIssuersEquityTypeAxis_A217D13FD08765FFA2B8AF31C81B0AA9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OptionIndexedToIssuersEquityTypeDomain" xlink:label="loc_us-gaap_OptionIndexedToIssuersEquityTypeDomain_FBE68EA8DE06F13EB092AF31C81C6EAC_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_OptionIndexedToIssuersEquityTypeAxis_A217D13FD08765FFA2B8AF31C81B0AA9" xlink:to="loc_us-gaap_OptionIndexedToIssuersEquityTypeDomain_FBE68EA8DE06F13EB092AF31C81C6EAC_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OptionIndexedToIssuersEquityTypeDomain" xlink:label="loc_us-gaap_OptionIndexedToIssuersEquityTypeDomain_FBE68EA8DE06F13EB092AF31C81C6EAC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_OptionIndexedToIssuersEquityTypeAxis_A217D13FD08765FFA2B8AF31C81B0AA9" xlink:to="loc_us-gaap_OptionIndexedToIssuersEquityTypeDomain_FBE68EA8DE06F13EB092AF31C81C6EAC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CallOptionMember" xlink:label="loc_us-gaap_CallOptionMember_D84A938F122CD03CC989AF31C81C559D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OptionIndexedToIssuersEquityTypeDomain_FBE68EA8DE06F13EB092AF31C81C6EAC" xlink:to="loc_us-gaap_CallOptionMember_D84A938F122CD03CC989AF31C81C559D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_AA4497FD1A62B266572CAF31C81C2910" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_290D79FDAA2DFB437C7BAF31C81BAAE9" xlink:to="loc_us-gaap_RangeAxis_AA4497FD1A62B266572CAF31C81C2910" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_0D5F01D1F7A344532DB5AF31C81C597A_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_AA4497FD1A62B266572CAF31C81C2910" xlink:to="loc_us-gaap_RangeMember_0D5F01D1F7A344532DB5AF31C81C597A_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_0D5F01D1F7A344532DB5AF31C81C597A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_AA4497FD1A62B266572CAF31C81C2910" xlink:to="loc_us-gaap_RangeMember_0D5F01D1F7A344532DB5AF31C81C597A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_F559C0EF8824CF4EBCAFAF31C81C9188" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_0D5F01D1F7A344532DB5AF31C81C597A" xlink:to="loc_us-gaap_MinimumMember_F559C0EF8824CF4EBCAFAF31C81C9188" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_4625C9BE80CC2611CCF9AF31C81C4BBA" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_0D5F01D1F7A344532DB5AF31C81C597A" xlink:to="loc_us-gaap_MaximumMember_4625C9BE80CC2611CCF9AF31C81C4BBA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByLiabilityClassAxis" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_5AAE11CDF7FFBF9AF27AAF31C81CB448" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_290D79FDAA2DFB437C7BAF31C81BAAE9" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_5AAE11CDF7FFBF9AF27AAF31C81CB448" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_02EA3EC143A78B3E4B12AF31C81CD3F1_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_5AAE11CDF7FFBF9AF27AAF31C81CB448" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_02EA3EC143A78B3E4B12AF31C81CD3F1_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_02EA3EC143A78B3E4B12AF31C81CD3F1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_5AAE11CDF7FFBF9AF27AAF31C81CB448" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_02EA3EC143A78B3E4B12AF31C81CD3F1" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_ContingentConsiderationMember" xlink:label="loc_dgx_ContingentConsiderationMember_2BB30DDD57A5CDC16893AF31C81C7FCD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_02EA3EC143A78B3E4B12AF31C81CD3F1" xlink:to="loc_dgx_ContingentConsiderationMember_2BB30DDD57A5CDC16893AF31C81C7FCD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_54164BBE918DBE115DA3AF31C81C5B6F" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_290D79FDAA2DFB437C7BAF31C81BAAE9" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_54164BBE918DBE115DA3AF31C81C5B6F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_390C1ACEC2CC6627D1D2AF31C81C412D_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_54164BBE918DBE115DA3AF31C81C5B6F" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_390C1ACEC2CC6627D1D2AF31C81C412D_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_390C1ACEC2CC6627D1D2AF31C81C412D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_54164BBE918DBE115DA3AF31C81C5B6F" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_390C1ACEC2CC6627D1D2AF31C81C412D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherOperatingIncomeExpenseMember" xlink:label="loc_us-gaap_OtherOperatingIncomeExpenseMember_7096CF322BB238D9670BAF31C81C3C90" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_390C1ACEC2CC6627D1D2AF31C81C412D" xlink:to="loc_us-gaap_OtherOperatingIncomeExpenseMember_7096CF322BB238D9670BAF31C81C3C90" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_PercentageOfEquityInterestExpectedToBeDivested" xlink:label="loc_dgx_PercentageOfEquityInterestExpectedToBeDivested_0D601E9CA71AB9AAEF52AF31C81C28DA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_20059A9DA172EC6159E1AF31C81C26E2" xlink:to="loc_dgx_PercentageOfEquityInterestExpectedToBeDivested_0D601E9CA71AB9AAEF52AF31C81C28DA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners" xlink:label="loc_us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners_88725F3BDEB00C1E58FDAF31C81C98D1" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_20059A9DA172EC6159E1AF31C81C26E2" xlink:to="loc_us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners_88725F3BDEB00C1E58FDAF31C81C98D1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferred1" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_E807DB773196182CCF0EAF31C81C6F17" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_20059A9DA172EC6159E1AF31C81C26E2" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_E807DB773196182CCF0EAF31C81C6F17" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProceedsFromMinorityShareholders" xlink:label="loc_us-gaap_ProceedsFromMinorityShareholders_051FB4EBA65634579A5AAF31C81CBCC7" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_20059A9DA172EC6159E1AF31C81C26E2" xlink:to="loc_us-gaap_ProceedsFromMinorityShareholders_051FB4EBA65634579A5AAF31C81CBCC7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_5910E026936969774CF1AF31C81C028D" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_20059A9DA172EC6159E1AF31C81C26E2" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_5910E026936969774CF1AF31C81C028D" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_Fairvalueinputsprobabilityofoccurrence" xlink:label="loc_dgx_Fairvalueinputsprobabilityofoccurrence_C633A5490C504B285503AF31C81CCB7E" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_20059A9DA172EC6159E1AF31C81C26E2" xlink:to="loc_dgx_Fairvalueinputsprobabilityofoccurrence_C633A5490C504B285503AF31C81CCB7E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueInputsDiscountRate" xlink:label="loc_us-gaap_FairValueInputsDiscountRate_DE4292C7FF4C9E7B7324AF31C81DD7B3" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_20059A9DA172EC6159E1AF31C81C26E2" xlink:to="loc_us-gaap_FairValueInputsDiscountRate_DE4292C7FF4C9E7B7324AF31C81DD7B3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_B33854425CEA18410C62AF31C81D8637" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_20059A9DA172EC6159E1AF31C81C26E2" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_B33854425CEA18410C62AF31C81D8637" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_7E7B933AF383921C9C99AF31C81DF285" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_20059A9DA172EC6159E1AF31C81C26E2" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_7E7B933AF383921C9C99AF31C81DF285" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentFairValue" xlink:label="loc_us-gaap_DebtInstrumentFairValue_38A3A53D2C1D6BA4899FAF31C81D82EF" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_20059A9DA172EC6159E1AF31C81C26E2" xlink:to="loc_us-gaap_DebtInstrumentFairValue_38A3A53D2C1D6BA4899FAF31C81D82EF" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_DebtInstrumentFairValueInExcessOfCarryingValueOfDebt" xlink:label="loc_dgx_DebtInstrumentFairValueInExcessOfCarryingValueOfDebt_485E12104A5B1B67C573AF4E3077CE7D" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_20059A9DA172EC6159E1AF31C81C26E2" xlink:to="loc_dgx_DebtInstrumentFairValueInExcessOfCarryingValueOfDebt_485E12104A5B1B67C573AF4E3077CE7D" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/FairValueMeasurementsRecognizedAssetsAndLiabilitiesAtFairValueDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_5CF33E58DA768ABCFF5E8B4CDA73B01C" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByBalanceSheetGroupingTable" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_9CDC79CE00D5CA6BDE6D8B4CDA6CECAC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_5CF33E58DA768ABCFF5E8B4CDA73B01C" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_9CDC79CE00D5CA6BDE6D8B4CDA6CECAC" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_FCF41A68D1072D7EC9588B4CDA726EE4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_9CDC79CE00D5CA6BDE6D8B4CDA6CECAC" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_FCF41A68D1072D7EC9588B4CDA726EE4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_9CBBA2B3B6773FEAD37C8B4CDA72182C_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_FCF41A68D1072D7EC9588B4CDA726EE4" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_9CBBA2B3B6773FEAD37C8B4CDA72182C_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_9CBBA2B3B6773FEAD37C8B4CDA72182C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_FCF41A68D1072D7EC9588B4CDA726EE4" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_9CBBA2B3B6773FEAD37C8B4CDA72182C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_0CDA60C40C288653CAF18B4CDA72C3A7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_9CBBA2B3B6773FEAD37C8B4CDA72182C" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_0CDA60C40C288653CAF18B4CDA72C3A7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeByNatureAxis" xlink:label="loc_us-gaap_DerivativeByNatureAxis_3D107C7C51A868906D148B4CDA726704" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_9CDC79CE00D5CA6BDE6D8B4CDA6CECAC" xlink:to="loc_us-gaap_DerivativeByNatureAxis_3D107C7C51A868906D148B4CDA726704" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeNameDomain" xlink:label="loc_us-gaap_DerivativeNameDomain_D4F4B29F8DA297A22E578B4CDA722B34_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeByNatureAxis_3D107C7C51A868906D148B4CDA726704" xlink:to="loc_us-gaap_DerivativeNameDomain_D4F4B29F8DA297A22E578B4CDA722B34_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeNameDomain" xlink:label="loc_us-gaap_DerivativeNameDomain_D4F4B29F8DA297A22E578B4CDA722B34" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeByNatureAxis_3D107C7C51A868906D148B4CDA726704" xlink:to="loc_us-gaap_DerivativeNameDomain_D4F4B29F8DA297A22E578B4CDA722B34" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InterestRateSwapMember" xlink:label="loc_us-gaap_InterestRateSwapMember_A7B702E888C43DF075968B4CDA730312" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeNameDomain_D4F4B29F8DA297A22E578B4CDA722B34" xlink:to="loc_us-gaap_InterestRateSwapMember_A7B702E888C43DF075968B4CDA730312" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PutOptionMember" xlink:label="loc_us-gaap_PutOptionMember_DC9D78F1485D687915438B4CDA735EEA" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeNameDomain_D4F4B29F8DA297A22E578B4CDA722B34" xlink:to="loc_us-gaap_PutOptionMember_DC9D78F1485D687915438B4CDA735EEA" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_ForwardStartingInterestRateSwapsMember" xlink:label="loc_dgx_ForwardStartingInterestRateSwapsMember_D52298C42727F3D908D88B4CDA73A9AB" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeNameDomain_D4F4B29F8DA297A22E578B4CDA722B34" xlink:to="loc_dgx_ForwardStartingInterestRateSwapsMember_D52298C42727F3D908D88B4CDA73A9AB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CallOptionMember" xlink:label="loc_us-gaap_CallOptionMember_E5020C22DFFE8069D6F98B4CDA739D49" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeNameDomain_D4F4B29F8DA297A22E578B4CDA722B34" xlink:to="loc_us-gaap_CallOptionMember_E5020C22DFFE8069D6F98B4CDA739D49" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_88EC709BD1A463069FC28B4CDA739420" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_9CDC79CE00D5CA6BDE6D8B4CDA6CECAC" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_88EC709BD1A463069FC28B4CDA739420" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_901A9ADC2D2571A85DFA8B4CDA7397AE_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_88EC709BD1A463069FC28B4CDA739420" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_901A9ADC2D2571A85DFA8B4CDA7397AE_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_901A9ADC2D2571A85DFA8B4CDA7397AE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_88EC709BD1A463069FC28B4CDA739420" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_901A9ADC2D2571A85DFA8B4CDA7397AE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_D263606414A53120AD8C8B4CDA73FAB3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_901A9ADC2D2571A85DFA8B4CDA7397AE" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_D263606414A53120AD8C8B4CDA73FAB3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_EC177790E0DAE0F237D08B4CDA730947" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_901A9ADC2D2571A85DFA8B4CDA7397AE" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_EC177790E0DAE0F237D08B4CDA730947" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_ECC665EB99D880454F648B4CDA73D8F3" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_901A9ADC2D2571A85DFA8B4CDA7397AE" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_ECC665EB99D880454F648B4CDA73D8F3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TradingSecurities" xlink:label="loc_us-gaap_TradingSecurities_19ECAAC42CC3893AB9068B4CDA73BD08" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_5CF33E58DA768ABCFF5E8B4CDA73B01C" xlink:to="loc_us-gaap_TradingSecurities_19ECAAC42CC3893AB9068B4CDA73BD08" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CashSurrenderValueFairValueDisclosure" xlink:label="loc_us-gaap_CashSurrenderValueFairValueDisclosure_C79D1FE749EBC9A8BA968B4CDA73E710" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_5CF33E58DA768ABCFF5E8B4CDA73B01C" xlink:to="loc_us-gaap_CashSurrenderValueFairValueDisclosure_C79D1FE749EBC9A8BA968B4CDA73E710" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeAssets" xlink:label="loc_us-gaap_DerivativeAssets_9644BB207EE3EDEFE8AD8B4CDA78DAA1" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_5CF33E58DA768ABCFF5E8B4CDA73B01C" xlink:to="loc_us-gaap_DerivativeAssets_9644BB207EE3EDEFE8AD8B4CDA78DAA1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesEquitySecuritiesNoncurrent" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesEquitySecuritiesNoncurrent_578726AEEFEB0C15E5208B4CDA7816A3" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_5CF33E58DA768ABCFF5E8B4CDA73B01C" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesEquitySecuritiesNoncurrent_578726AEEFEB0C15E5208B4CDA7816A3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AssetsFairValueDisclosureRecurring" xlink:label="loc_us-gaap_AssetsFairValueDisclosureRecurring_5ABD76D04EA8052181A48B4CDA79BEB9" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_5CF33E58DA768ABCFF5E8B4CDA73B01C" xlink:to="loc_us-gaap_AssetsFairValueDisclosureRecurring_5ABD76D04EA8052181A48B4CDA79BEB9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredCompensationLiabilityClassifiedNoncurrent" xlink:label="loc_us-gaap_DeferredCompensationLiabilityClassifiedNoncurrent_9C2CDF13A2406B7F2E7E8B4CDA79F63D" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_5CF33E58DA768ABCFF5E8B4CDA73B01C" xlink:to="loc_us-gaap_DeferredCompensationLiabilityClassifiedNoncurrent_9C2CDF13A2406B7F2E7E8B4CDA79F63D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_89010E629C29E9E29DBD8B4CDA79FDA6" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_5CF33E58DA768ABCFF5E8B4CDA73B01C" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_89010E629C29E9E29DBD8B4CDA79FDA6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeLiabilities" xlink:label="loc_us-gaap_DerivativeLiabilities_433526518DC8DF447EB58B4CDA79D4AC" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_5CF33E58DA768ABCFF5E8B4CDA73B01C" xlink:to="loc_us-gaap_DerivativeLiabilities_433526518DC8DF447EB58B4CDA79D4AC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LiabilitiesFairValueDisclosureRecurring" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosureRecurring_1FDD2EA55E283A1562028B4CDA79504E" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_5CF33E58DA768ABCFF5E8B4CDA73B01C" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosureRecurring_1FDD2EA55E283A1562028B4CDA79504E" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/FairValueMeasurementsReconciliationOfBeginningAndEndingBalancesOfAssetsAndLiabilitiesUnobservableInputsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems" xlink:label="loc_us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_4C17C92018AC871CFD9490522F7A09A6" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:label="loc_us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTable_1F4D7EEF342A6854DD0F90522F77FC56" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_4C17C92018AC871CFD9490522F7A09A6" xlink:to="loc_us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTable_1F4D7EEF342A6854DD0F90522F77FC56" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_DB6D565A5DD87249D52990522F78BA79" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTable_1F4D7EEF342A6854DD0F90522F77FC56" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_DB6D565A5DD87249D52990522F78BA79" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2515AA8AFEF773AF86AD90522F78055E_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_DB6D565A5DD87249D52990522F78BA79" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2515AA8AFEF773AF86AD90522F78055E_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2515AA8AFEF773AF86AD90522F78055E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_DB6D565A5DD87249D52990522F78BA79" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2515AA8AFEF773AF86AD90522F78055E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_CC3118AC8D9DF63A5B6490522F78647D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2515AA8AFEF773AF86AD90522F78055E" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_CC3118AC8D9DF63A5B6490522F78647D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByAssetClassAxis" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_91EA4BE627B89EBD520D90522F78CB79" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTable_1F4D7EEF342A6854DD0F90522F77FC56" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_91EA4BE627B89EBD520D90522F78CB79" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_58AB8798EF644C94720E90522F787F4D_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_91EA4BE627B89EBD520D90522F78CB79" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_58AB8798EF644C94720E90522F787F4D_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_58AB8798EF644C94720E90522F787F4D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_91EA4BE627B89EBD520D90522F78CB79" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_58AB8798EF644C94720E90522F787F4D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PutOptionMember" xlink:label="loc_us-gaap_PutOptionMember_ABE0DD5AC3C7126D7C8690522F780214" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_58AB8798EF644C94720E90522F787F4D" xlink:to="loc_us-gaap_PutOptionMember_ABE0DD5AC3C7126D7C8690522F780214" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByLiabilityClassAxis" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_92B39263DE71AD42E59090522F799460" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTable_1F4D7EEF342A6854DD0F90522F77FC56" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_92B39263DE71AD42E59090522F799460" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_11D5B9CA21512485944690522F7977A9_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_92B39263DE71AD42E59090522F799460" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_11D5B9CA21512485944690522F7977A9_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_11D5B9CA21512485944690522F7977A9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_92B39263DE71AD42E59090522F799460" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_11D5B9CA21512485944690522F7977A9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CallOptionMember" xlink:label="loc_us-gaap_CallOptionMember_DB31F77634B74B9D266590522F799210" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_11D5B9CA21512485944690522F7977A9" xlink:to="loc_us-gaap_CallOptionMember_DB31F77634B74B9D266590522F799210" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_ContingentConsiderationMember" xlink:label="loc_dgx_ContingentConsiderationMember_6C2D51A98E79207F602290522F791CDE" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_11D5B9CA21512485944690522F7977A9" xlink:to="loc_dgx_ContingentConsiderationMember_6C2D51A98E79207F602290522F791CDE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_65ABF8BF3F624E097B1690522F7AC399" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_4C17C92018AC871CFD9490522F7A09A6" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_65ABF8BF3F624E097B1690522F7AC399" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue_53A59E4D8F85A76AC2C590522F7A8775" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_65ABF8BF3F624E097B1690522F7AC399" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue_53A59E4D8F85A76AC2C590522F7A8775" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements_7EF3029C499614BAE39990522F7B7B3B" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_65ABF8BF3F624E097B1690522F7AC399" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements_7EF3029C499614BAE39990522F7B7B3B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1_84D5A402D4D9C28D6C6090522F7B3B72" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_65ABF8BF3F624E097B1690522F7AC399" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1_84D5A402D4D9C28D6C6090522F7B3B72" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_D146B7783FBD426DB46F90522F7B6F90" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_4C17C92018AC871CFD9490522F7A09A6" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_D146B7783FBD426DB46F90522F7B6F90" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_98836A1C533C6E80F52590522F7B7A72" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_D146B7783FBD426DB46F90522F7B6F90" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_98836A1C533C6E80F52590522F7B7A72" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_196090CD1F0661D33CEB90522F7CB60C" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_D146B7783FBD426DB46F90522F7B6F90" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_196090CD1F0661D33CEB90522F7CB60C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_5CC4BD53FF3521269A9890522F7C0CC8" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_D146B7783FBD426DB46F90522F7B6F90" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_5CC4BD53FF3521269A9890522F7C0CC8" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/FairValueMeasurementsTables" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/FinancialInstruments" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/FinancialInstrumentsNarrativeDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeLineItems" xlink:label="loc_us-gaap_DerivativeLineItems_94F2AF9588FBE53BA23128FD57C52CE1" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeTable" xlink:label="loc_us-gaap_DerivativeTable_F51B7ADF721486B7FA9928FD57C54BCD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_94F2AF9588FBE53BA23128FD57C52CE1" xlink:to="loc_us-gaap_DerivativeTable_F51B7ADF721486B7FA9928FD57C54BCD" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_012A5A0641FB1ED21A7E28FD57C5C7D3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_F51B7ADF721486B7FA9928FD57C54BCD" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_012A5A0641FB1ED21A7E28FD57C5C7D3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingRelationshipDomain" xlink:label="loc_us-gaap_HedgingRelationshipDomain_B002AB94171D2FDF544B28FD57C5465E_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_012A5A0641FB1ED21A7E28FD57C5C7D3" xlink:to="loc_us-gaap_HedgingRelationshipDomain_B002AB94171D2FDF544B28FD57C5465E_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingRelationshipDomain" xlink:label="loc_us-gaap_HedgingRelationshipDomain_B002AB94171D2FDF544B28FD57C5465E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_012A5A0641FB1ED21A7E28FD57C5C7D3" xlink:to="loc_us-gaap_HedgingRelationshipDomain_B002AB94171D2FDF544B28FD57C5465E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CashFlowHedgingMember" xlink:label="loc_us-gaap_CashFlowHedgingMember_6BBFC0765D088061CD9D28FD57C55C59" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingRelationshipDomain_B002AB94171D2FDF544B28FD57C5465E" xlink:to="loc_us-gaap_CashFlowHedgingMember_6BBFC0765D088061CD9D28FD57C55C59" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_EconomicHedgesMember" xlink:label="loc_dgx_EconomicHedgesMember_38FB97DDF3825827281828FD57C55110" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingRelationshipDomain_B002AB94171D2FDF544B28FD57C5465E" xlink:to="loc_dgx_EconomicHedgesMember_38FB97DDF3825827281828FD57C55110" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeByNatureAxis" xlink:label="loc_us-gaap_DerivativeByNatureAxis_E6FB078DC3490BF7E05F28FD57C5D319" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_F51B7ADF721486B7FA9928FD57C54BCD" xlink:to="loc_us-gaap_DerivativeByNatureAxis_E6FB078DC3490BF7E05F28FD57C5D319" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeNameDomain" xlink:label="loc_us-gaap_DerivativeNameDomain_0F1351A3B0BFC53A22B328FD57C5F583_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeByNatureAxis_E6FB078DC3490BF7E05F28FD57C5D319" xlink:to="loc_us-gaap_DerivativeNameDomain_0F1351A3B0BFC53A22B328FD57C5F583_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeNameDomain" xlink:label="loc_us-gaap_DerivativeNameDomain_0F1351A3B0BFC53A22B328FD57C5F583" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeByNatureAxis_E6FB078DC3490BF7E05F28FD57C5D319" xlink:to="loc_us-gaap_DerivativeNameDomain_0F1351A3B0BFC53A22B328FD57C5F583" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_ForwardStartingInterestRateSwapsMember" xlink:label="loc_dgx_ForwardStartingInterestRateSwapsMember_F595D3D719902A5CEA5328FD57C5407B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeNameDomain_0F1351A3B0BFC53A22B328FD57C5F583" xlink:to="loc_dgx_ForwardStartingInterestRateSwapsMember_F595D3D719902A5CEA5328FD57C5407B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TreasuryLockMember" xlink:label="loc_us-gaap_TreasuryLockMember_1610CF3206D776A06AA228FD57C57411" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeNameDomain_0F1351A3B0BFC53A22B328FD57C5F583" xlink:to="loc_us-gaap_TreasuryLockMember_1610CF3206D776A06AA228FD57C57411" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_ReverseTreasuryLocksMember" xlink:label="loc_dgx_ReverseTreasuryLocksMember_B5CF77ECD5F6A0153FB628FD57C50F02" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeNameDomain_0F1351A3B0BFC53A22B328FD57C5F583" xlink:to="loc_dgx_ReverseTreasuryLocksMember_B5CF77ECD5F6A0153FB628FD57C50F02" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ForwardContractsMember" xlink:label="loc_us-gaap_ForwardContractsMember_54840A765300251E74FE28FD57C5E77D" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeNameDomain_0F1351A3B0BFC53A22B328FD57C5F583" xlink:to="loc_us-gaap_ForwardContractsMember_54840A765300251E74FE28FD57C5E77D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationAxis" xlink:label="loc_us-gaap_HedgingDesignationAxis_A669224159818BDF409228FD57C5CE01" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_F51B7ADF721486B7FA9928FD57C54BCD" xlink:to="loc_us-gaap_HedgingDesignationAxis_A669224159818BDF409228FD57C5CE01" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="loc_us-gaap_HedgingDesignationDomain_74AB95FCE036535DA70C28FD57C513F8_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_A669224159818BDF409228FD57C5CE01" xlink:to="loc_us-gaap_HedgingDesignationDomain_74AB95FCE036535DA70C28FD57C513F8_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="loc_us-gaap_HedgingDesignationDomain_74AB95FCE036535DA70C28FD57C513F8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_A669224159818BDF409228FD57C5CE01" xlink:to="loc_us-gaap_HedgingDesignationDomain_74AB95FCE036535DA70C28FD57C513F8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NondesignatedMember" xlink:label="loc_us-gaap_NondesignatedMember_A9CA5921D786B4D1C2D128FD57C59381" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_74AB95FCE036535DA70C28FD57C513F8" xlink:to="loc_us-gaap_NondesignatedMember_A9CA5921D786B4D1C2D128FD57C59381" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_E5C9E6AD5ADE021ADE5028FD57C54040" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_F51B7ADF721486B7FA9928FD57C54BCD" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_E5C9E6AD5ADE021ADE5028FD57C54040" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_435B5C593BCC937AB98828FD57C56DD9_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_E5C9E6AD5ADE021ADE5028FD57C54040" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_435B5C593BCC937AB98828FD57C56DD9_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_435B5C593BCC937AB98828FD57C56DD9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_E5C9E6AD5ADE021ADE5028FD57C54040" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_435B5C593BCC937AB98828FD57C56DD9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherOperatingIncomeExpenseMember" xlink:label="loc_us-gaap_OtherOperatingIncomeExpenseMember_36A7D549C4FDB4D2F0B328FD57C589B4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_435B5C593BCC937AB98828FD57C56DD9" xlink:to="loc_us-gaap_OtherOperatingIncomeExpenseMember_36A7D549C4FDB4D2F0B328FD57C589B4" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_TriggeringEventAxis" xlink:label="loc_dgx_TriggeringEventAxis_9953F677AFE6023ECC5A28FD57C5FA78" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_F51B7ADF721486B7FA9928FD57C54BCD" xlink:to="loc_dgx_TriggeringEventAxis_9953F677AFE6023ECC5A28FD57C5FA78" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_TriggeringEventDomain" xlink:label="loc_dgx_TriggeringEventDomain_DF123DF190616A61DC7728FD57C5E3CA_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dgx_TriggeringEventAxis_9953F677AFE6023ECC5A28FD57C5FA78" xlink:to="loc_dgx_TriggeringEventDomain_DF123DF190616A61DC7728FD57C5E3CA_default" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_TriggeringEventDomain" xlink:label="loc_dgx_TriggeringEventDomain_DF123DF190616A61DC7728FD57C5E3CA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dgx_TriggeringEventAxis_9953F677AFE6023ECC5A28FD57C5FA78" xlink:to="loc_dgx_TriggeringEventDomain_DF123DF190616A61DC7728FD57C5E3CA" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_TobeRetiredinQ1andQ22015Member" xlink:label="loc_dgx_TobeRetiredinQ1andQ22015Member_41275176A43BA6634C5928FD57C5E1BC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_TriggeringEventDomain_DF123DF190616A61DC7728FD57C5E3CA" xlink:to="loc_dgx_TobeRetiredinQ1andQ22015Member_41275176A43BA6634C5928FD57C5E1BC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/invest/2013/invest-2013-01-31.xsd#invest_DerivativeNotionalAmount" xlink:label="loc_invest_DerivativeNotionalAmount_FB79CEDDB24ABC052E4628FD57C510B4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_94F2AF9588FBE53BA23128FD57C52CE1" xlink:to="loc_invest_DerivativeNotionalAmount_FB79CEDDB24ABC052E4628FD57C510B4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_UnrealizedGainLossOnInterestRateCashFlowHedgesPretaxAccumulatedOtherComprehensiveIncomeLoss" xlink:label="loc_us-gaap_UnrealizedGainLossOnInterestRateCashFlowHedgesPretaxAccumulatedOtherComprehensiveIncomeLoss_11515F58119191849EB728FD57C514A0" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_94F2AF9588FBE53BA23128FD57C52CE1" xlink:to="loc_us-gaap_UnrealizedGainLossOnInterestRateCashFlowHedgesPretaxAccumulatedOtherComprehensiveIncomeLoss_11515F58119191849EB728FD57C514A0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGain" xlink:label="loc_us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGain_1CCCD6AA1D670B0004C028FD57C5EE9A" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_94F2AF9588FBE53BA23128FD57C52CE1" xlink:to="loc_us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGain_1CCCD6AA1D670B0004C028FD57C5EE9A" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_AccumulatedNetGainLossfromDesignatedorQualifyingCashFlowHedges" xlink:label="loc_dgx_AccumulatedNetGainLossfromDesignatedorQualifyingCashFlowHedges_3806E498E00F7352F1C428FD57C54E7C" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_94F2AF9588FBE53BA23128FD57C52CE1" xlink:to="loc_dgx_AccumulatedNetGainLossfromDesignatedorQualifyingCashFlowHedges_3806E498E00F7352F1C428FD57C54E7C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths" xlink:label="loc_us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths_7AC3B5446A0FB5392D2228FD57C535D4" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_94F2AF9588FBE53BA23128FD57C52CE1" xlink:to="loc_us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths_7AC3B5446A0FB5392D2228FD57C535D4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentFaceAmount" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_8F44BB9943C129F9F0FA28FD57C505DC" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_94F2AF9588FBE53BA23128FD57C52CE1" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_8F44BB9943C129F9F0FA28FD57C505DC" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/FinancialInstrumentsSummaryOfFairValueOfDerivativesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeLineItems" xlink:label="loc_us-gaap_DerivativeLineItems_C90C294EAE943590704990522D11060A" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeTable" xlink:label="loc_us-gaap_DerivativeTable_9686DF7261B3A7530CF890522D09E9B3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_C90C294EAE943590704990522D11060A" xlink:to="loc_us-gaap_DerivativeTable_9686DF7261B3A7530CF890522D09E9B3" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationAxis" xlink:label="loc_us-gaap_HedgingDesignationAxis_0B67A069EAA1946D66C190522D0AD428" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_9686DF7261B3A7530CF890522D09E9B3" xlink:to="loc_us-gaap_HedgingDesignationAxis_0B67A069EAA1946D66C190522D0AD428" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="loc_us-gaap_HedgingDesignationDomain_6456F660439D0D8D9AD690522D0A2978_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_0B67A069EAA1946D66C190522D0AD428" xlink:to="loc_us-gaap_HedgingDesignationDomain_6456F660439D0D8D9AD690522D0A2978_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="loc_us-gaap_HedgingDesignationDomain_6456F660439D0D8D9AD690522D0A2978" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_0B67A069EAA1946D66C190522D0AD428" xlink:to="loc_us-gaap_HedgingDesignationDomain_6456F660439D0D8D9AD690522D0A2978" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DesignatedAsHedgingInstrumentMember" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_6D36F9E785DDD2BCD01A90522D0A2589" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_6456F660439D0D8D9AD690522D0A2978" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_6D36F9E785DDD2BCD01A90522D0A2589" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NondesignatedMember" xlink:label="loc_us-gaap_NondesignatedMember_3367DE07A0E4C2D5AECA90522D0AC817" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_6456F660439D0D8D9AD690522D0A2978" xlink:to="loc_us-gaap_NondesignatedMember_3367DE07A0E4C2D5AECA90522D0AC817" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_1C5965E5D0AFDD11E6B290522D0A999D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_9686DF7261B3A7530CF890522D09E9B3" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_1C5965E5D0AFDD11E6B290522D0A999D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_F683A8D31BDA6DC8571490522D0A8D27_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_1C5965E5D0AFDD11E6B290522D0A999D" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_F683A8D31BDA6DC8571490522D0A8D27_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_F683A8D31BDA6DC8571490522D0A8D27" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_1C5965E5D0AFDD11E6B290522D0A999D" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_F683A8D31BDA6DC8571490522D0A8D27" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherAssetsMember" xlink:label="loc_us-gaap_OtherAssetsMember_3779703702ABDA51F74590522D0AA124" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_F683A8D31BDA6DC8571490522D0A8D27" xlink:to="loc_us-gaap_OtherAssetsMember_3779703702ABDA51F74590522D0AA124" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherLiabilitiesMember" xlink:label="loc_us-gaap_OtherLiabilitiesMember_E9841CE4590DE00EFBED90522D0B7A8A" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_F683A8D31BDA6DC8571490522D0A8D27" xlink:to="loc_us-gaap_OtherLiabilitiesMember_E9841CE4590DE00EFBED90522D0B7A8A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember" xlink:label="loc_us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember_542E0BF668D8A7ACD2EF90522D0BACA5" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_F683A8D31BDA6DC8571490522D0A8D27" xlink:to="loc_us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember_542E0BF668D8A7ACD2EF90522D0BACA5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccountsPayableAndAccruedLiabilitiesMember" xlink:label="loc_us-gaap_AccountsPayableAndAccruedLiabilitiesMember_448460FEEDAD6837B95F90522D0BFB31" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_F683A8D31BDA6DC8571490522D0A8D27" xlink:to="loc_us-gaap_AccountsPayableAndAccruedLiabilitiesMember_448460FEEDAD6837B95F90522D0BFB31" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeByNatureAxis" xlink:label="loc_us-gaap_DerivativeByNatureAxis_94C54D3377CB2CB2443C90522D0BA4BA" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_9686DF7261B3A7530CF890522D09E9B3" xlink:to="loc_us-gaap_DerivativeByNatureAxis_94C54D3377CB2CB2443C90522D0BA4BA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeNameDomain" xlink:label="loc_us-gaap_DerivativeNameDomain_CF7443940F677D3E163590522D10876A_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeByNatureAxis_94C54D3377CB2CB2443C90522D0BA4BA" xlink:to="loc_us-gaap_DerivativeNameDomain_CF7443940F677D3E163590522D10876A_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeNameDomain" xlink:label="loc_us-gaap_DerivativeNameDomain_CF7443940F677D3E163590522D10876A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeByNatureAxis_94C54D3377CB2CB2443C90522D0BA4BA" xlink:to="loc_us-gaap_DerivativeNameDomain_CF7443940F677D3E163590522D10876A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InterestRateSwapMember" xlink:label="loc_us-gaap_InterestRateSwapMember_13ECB6EF40415403392490522D108594" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeNameDomain_CF7443940F677D3E163590522D10876A" xlink:to="loc_us-gaap_InterestRateSwapMember_13ECB6EF40415403392490522D108594" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ForwardContractsMember" xlink:label="loc_us-gaap_ForwardContractsMember_B6945C2F679C3B6FC3CC90522D1151EB" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeNameDomain_CF7443940F677D3E163590522D10876A" xlink:to="loc_us-gaap_ForwardContractsMember_B6945C2F679C3B6FC3CC90522D1151EB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PutOptionMember" xlink:label="loc_us-gaap_PutOptionMember_C8347F4EDE4CA6309BBD90522D11CB5D" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeNameDomain_CF7443940F677D3E163590522D10876A" xlink:to="loc_us-gaap_PutOptionMember_C8347F4EDE4CA6309BBD90522D11CB5D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CallOptionMember" xlink:label="loc_us-gaap_CallOptionMember_A41355030A177D55EF8990522D11D198" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeNameDomain_CF7443940F677D3E163590522D10876A" xlink:to="loc_us-gaap_CallOptionMember_A41355030A177D55EF8990522D11D198" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeFairValueOfDerivativeAsset" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_0E57283B4F48EA53E95090522D1113B6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_C90C294EAE943590704990522D11060A" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_0E57283B4F48EA53E95090522D1113B6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeFairValueOfDerivativeLiability" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_B36CC92BB3D709381F6C90522D113E29" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_C90C294EAE943590704990522D11060A" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_B36CC92BB3D709381F6C90522D113E29" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeAssetsLiabilitiesAtFairValueNet" xlink:label="loc_us-gaap_DerivativeAssetsLiabilitiesAtFairValueNet_5CBDB4114CBC5E55791490522D11D37B" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_C90C294EAE943590704990522D11060A" xlink:to="loc_us-gaap_DerivativeAssetsLiabilitiesAtFairValueNet_5CBDB4114CBC5E55791490522D11D37B" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/FinancialInstrumentsSummaryOfNotionalAmountsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeLineItems" xlink:label="loc_us-gaap_DerivativeLineItems_47BA9C7ADA1CDCEEC4DE28FD55E10701" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeTable" xlink:label="loc_us-gaap_DerivativeTable_46C2625E9C609DA286E028FD55E19C88" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_47BA9C7ADA1CDCEEC4DE28FD55E10701" xlink:to="loc_us-gaap_DerivativeTable_46C2625E9C609DA286E028FD55E19C88" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_628C1D98C4144310CF9C28FD55E19BF1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_46C2625E9C609DA286E028FD55E19C88" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_628C1D98C4144310CF9C28FD55E19BF1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingRelationshipDomain" xlink:label="loc_us-gaap_HedgingRelationshipDomain_46A01061D9FF3020211628FD55E1AED8_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_628C1D98C4144310CF9C28FD55E19BF1" xlink:to="loc_us-gaap_HedgingRelationshipDomain_46A01061D9FF3020211628FD55E1AED8_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingRelationshipDomain" xlink:label="loc_us-gaap_HedgingRelationshipDomain_46A01061D9FF3020211628FD55E1AED8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_628C1D98C4144310CF9C28FD55E19BF1" xlink:to="loc_us-gaap_HedgingRelationshipDomain_46A01061D9FF3020211628FD55E1AED8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueHedgingMember" xlink:label="loc_us-gaap_FairValueHedgingMember_C1E5FC58CA492176A64328FD55E11D7E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingRelationshipDomain_46A01061D9FF3020211628FD55E1AED8" xlink:to="loc_us-gaap_FairValueHedgingMember_C1E5FC58CA492176A64328FD55E11D7E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_2B5898569341794EFC3728FD55E1680C" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_46C2625E9C609DA286E028FD55E19C88" xlink:to="loc_us-gaap_DebtInstrumentAxis_2B5898569341794EFC3728FD55E1680C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_4587A50BA5CC13D1709A28FD55E16120_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_2B5898569341794EFC3728FD55E1680C" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_4587A50BA5CC13D1709A28FD55E16120_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_4587A50BA5CC13D1709A28FD55E16120" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_2B5898569341794EFC3728FD55E1680C" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_4587A50BA5CC13D1709A28FD55E16120" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_ThreePointTwoZeroPercentSeniorNotesdueApril2016Member" xlink:label="loc_dgx_ThreePointTwoZeroPercentSeniorNotesdueApril2016Member_7FCEEC211D9C2237E31228FD55E1B7EF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4587A50BA5CC13D1709A28FD55E16120" xlink:to="loc_dgx_ThreePointTwoZeroPercentSeniorNotesdueApril2016Member_7FCEEC211D9C2237E31228FD55E1B7EF" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_FourPointSevenFivePercentSeniorNotesDue2020Member" xlink:label="loc_dgx_FourPointSevenFivePercentSeniorNotesDue2020Member_0A33091AF9624B854F3628FD55E146B6" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4587A50BA5CC13D1709A28FD55E16120" xlink:to="loc_dgx_FourPointSevenFivePercentSeniorNotesDue2020Member_0A33091AF9624B854F3628FD55E146B6" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_FourPointSevenZeroPercentSeniorNotesdueApril2021Member" xlink:label="loc_dgx_FourPointSevenZeroPercentSeniorNotesdueApril2021Member_D5DEAA32C7F8A36476BB28FD55E10604" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4587A50BA5CC13D1709A28FD55E16120" xlink:to="loc_dgx_FourPointSevenZeroPercentSeniorNotesdueApril2021Member_D5DEAA32C7F8A36476BB28FD55E10604" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_FourpointtwofivepercentSeniorNotesDue2024Member" xlink:label="loc_dgx_FourpointtwofivepercentSeniorNotesDue2024Member_73225FE3E4C3421BE4A328FD55E17D8C" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4587A50BA5CC13D1709A28FD55E16120" xlink:to="loc_dgx_FourpointtwofivepercentSeniorNotesDue2024Member_73225FE3E4C3421BE4A328FD55E17D8C" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_ThreePointFiveZeroPercentSeniorNotesdueMarch2025Member" xlink:label="loc_dgx_ThreePointFiveZeroPercentSeniorNotesdueMarch2025Member_413BCD33A0AC6924720B28FD55E19176" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4587A50BA5CC13D1709A28FD55E16120" xlink:to="loc_dgx_ThreePointFiveZeroPercentSeniorNotesdueMarch2025Member_413BCD33A0AC6924720B28FD55E19176" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_VariableRateAxis" xlink:label="loc_us-gaap_VariableRateAxis_DC1CE4B26C7506FF02DB28FD55E10AC5" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_46C2625E9C609DA286E028FD55E19C88" xlink:to="loc_us-gaap_VariableRateAxis_DC1CE4B26C7506FF02DB28FD55E10AC5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_VariableRateDomain" xlink:label="loc_us-gaap_VariableRateDomain_870C5E7B97555055255C28FD55E1216A_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_DC1CE4B26C7506FF02DB28FD55E10AC5" xlink:to="loc_us-gaap_VariableRateDomain_870C5E7B97555055255C28FD55E1216A_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_VariableRateDomain" xlink:label="loc_us-gaap_VariableRateDomain_870C5E7B97555055255C28FD55E1216A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_DC1CE4B26C7506FF02DB28FD55E10AC5" xlink:to="loc_us-gaap_VariableRateDomain_870C5E7B97555055255C28FD55E1216A" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_SixMonthLondonInterbankOfferedRateLIBORMember" xlink:label="loc_dgx_SixMonthLondonInterbankOfferedRateLIBORMember_DD235637E8493A8DDC1F28FD55E143C4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_870C5E7B97555055255C28FD55E1216A" xlink:to="loc_dgx_SixMonthLondonInterbankOfferedRateLIBORMember_DD235637E8493A8DDC1F28FD55E143C4" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_OneMonthLondonInterbankOfferedRateLIBORMember" xlink:label="loc_dgx_OneMonthLondonInterbankOfferedRateLIBORMember_57991AD799CF7725E6C428FD55E143B9" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_870C5E7B97555055255C28FD55E1216A" xlink:to="loc_dgx_OneMonthLondonInterbankOfferedRateLIBORMember_57991AD799CF7725E6C428FD55E143B9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_7A8525D48B5EBA52CBB028FD55E1FCDD" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_46C2625E9C609DA286E028FD55E19C88" xlink:to="loc_us-gaap_RangeAxis_7A8525D48B5EBA52CBB028FD55E1FCDD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_0DFB4683AE63C64FC74628FD55E15AF8_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_7A8525D48B5EBA52CBB028FD55E1FCDD" xlink:to="loc_us-gaap_RangeMember_0DFB4683AE63C64FC74628FD55E15AF8_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_0DFB4683AE63C64FC74628FD55E15AF8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_7A8525D48B5EBA52CBB028FD55E1FCDD" xlink:to="loc_us-gaap_RangeMember_0DFB4683AE63C64FC74628FD55E15AF8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_10CFED9BB7D88E15866328FD55E11D81" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_0DFB4683AE63C64FC74628FD55E15AF8" xlink:to="loc_us-gaap_MinimumMember_10CFED9BB7D88E15866328FD55E11D81" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_45BDC9ABDA8BFD0A097028FD55E19812" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_0DFB4683AE63C64FC74628FD55E15AF8" xlink:to="loc_us-gaap_MaximumMember_45BDC9ABDA8BFD0A097028FD55E19812" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_0E46C8F75857DA17B30F28FD55E1144A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_47BA9C7ADA1CDCEEC4DE28FD55E10701" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_0E46C8F75857DA17B30F28FD55E1144A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentMaturityDate" xlink:label="loc_us-gaap_DebtInstrumentMaturityDate_6916C4DC5E7C91A93C2028FD55E134A3" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_47BA9C7ADA1CDCEEC4DE28FD55E10701" xlink:to="loc_us-gaap_DebtInstrumentMaturityDate_6916C4DC5E7C91A93C2028FD55E134A3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeForwardInterestRate" xlink:label="loc_us-gaap_DerivativeForwardInterestRate_A94E50F65498CA275B9828FD55E17812" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_47BA9C7ADA1CDCEEC4DE28FD55E10701" xlink:to="loc_us-gaap_DerivativeForwardInterestRate_A94E50F65498CA275B9828FD55E17812" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/invest/2013/invest-2013-01-31.xsd#invest_DerivativeNotionalAmount" xlink:label="loc_invest_DerivativeNotionalAmount_9DAD3EA05A4C76E6517228FD55E14735" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_47BA9C7ADA1CDCEEC4DE28FD55E10701" xlink:to="loc_invest_DerivativeNotionalAmount_9DAD3EA05A4C76E6517228FD55E14735" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/FinancialInstrumentsTables" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/GoodwillAndIntangibleAssets" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/GoodwillAndIntangibleAssetsDetails" xlink:type="extended">
    <loc xlink:href="dgx-20150930.xsd#dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems" xlink:label="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:type="locator" />
    <loc xlink:href="dgx-20150930.xsd#dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsTable" xlink:label="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsTable_8DF9D67EC3B80624AF62ABCA738E3AEE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsTable_8DF9D67EC3B80624AF62ABCA738E3AEE" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_7B189556025EB107A921ABCA738E26BB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsTable_8DF9D67EC3B80624AF62ABCA738E3AEE" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_7B189556025EB107A921ABCA738E26BB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_26D389E2C4CAAD6D148CABCA738EEDC1_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_7B189556025EB107A921ABCA738E26BB" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_26D389E2C4CAAD6D148CABCA738EEDC1_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_26D389E2C4CAAD6D148CABCA738EEDC1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_7B189556025EB107A921ABCA738E26BB" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_26D389E2C4CAAD6D148CABCA738EEDC1" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_SolstasSummitHealthandStewardMember" xlink:label="loc_dgx_SolstasSummitHealthandStewardMember_62A839F57711FE0CD98EABCA738F144A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_26D389E2C4CAAD6D148CABCA738EEDC1" xlink:to="loc_dgx_SolstasSummitHealthandStewardMember_62A839F57711FE0CD98EABCA738F144A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_529A63160EFF23DDE6ACABCA738FDC11" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsTable_8DF9D67EC3B80624AF62ABCA738E3AEE" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_529A63160EFF23DDE6ACABCA738FDC11" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_DB97E4B3E5E696F58FFDABCA738F73CB_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_529A63160EFF23DDE6ACABCA738FDC11" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_DB97E4B3E5E696F58FFDABCA738F73CB_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_DB97E4B3E5E696F58FFDABCA738F73CB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_529A63160EFF23DDE6ACABCA738FDC11" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_DB97E4B3E5E696F58FFDABCA738F73CB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CustomerRelationshipsMember" xlink:label="loc_us-gaap_CustomerRelationshipsMember_DD7BB0CC61CBB9EAAC06ABCA73900C7B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_DB97E4B3E5E696F58FFDABCA738F73CB" xlink:to="loc_us-gaap_CustomerRelationshipsMember_DD7BB0CC61CBB9EAAC06ABCA73900C7B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NoncompeteAgreementsMember" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_3E24737C7E55A0CBFEEAABCA73908255" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_DB97E4B3E5E696F58FFDABCA738F73CB" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_3E24737C7E55A0CBFEEAABCA73908255" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_UnpatentedTechnologyMember" xlink:label="loc_us-gaap_UnpatentedTechnologyMember_2FF55B9B374657237FA4ABCA73902DEE" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_DB97E4B3E5E696F58FFDABCA738F73CB" xlink:to="loc_us-gaap_UnpatentedTechnologyMember_2FF55B9B374657237FA4ABCA73902DEE" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_OtherIntangibleAssetsSubjectToAmortizationMember" xlink:label="loc_dgx_OtherIntangibleAssetsSubjectToAmortizationMember_D03D6218E230A1D7CD2AABCA7390237E" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_DB97E4B3E5E696F58FFDABCA738F73CB" xlink:to="loc_dgx_OtherIntangibleAssetsSubjectToAmortizationMember_D03D6218E230A1D7CD2AABCA7390237E" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_TotalAmortizingIntangibleAssetsMember" xlink:label="loc_dgx_TotalAmortizingIntangibleAssetsMember_0FB50D09B3398D71161AABCA739008CE" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_DB97E4B3E5E696F58FFDABCA738F73CB" xlink:to="loc_dgx_TotalAmortizingIntangibleAssetsMember_0FB50D09B3398D71161AABCA739008CE" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_NoncompeteAgreementsFullyAmortizedMember" xlink:label="loc_dgx_NoncompeteAgreementsFullyAmortizedMember_3D8C82FDB934907A3831ABCA7390C4AB" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_DB97E4B3E5E696F58FFDABCA738F73CB" xlink:to="loc_dgx_NoncompeteAgreementsFullyAmortizedMember_3D8C82FDB934907A3831ABCA7390C4AB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_B20D098EAB6AEC49B0D8ABCA73900059" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsTable_8DF9D67EC3B80624AF62ABCA738E3AEE" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_B20D098EAB6AEC49B0D8ABCA73900059" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_7A45380584A10ECE2E10ABCA7390E832_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_B20D098EAB6AEC49B0D8ABCA73900059" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_7A45380584A10ECE2E10ABCA7390E832_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_7A45380584A10ECE2E10ABCA7390E832" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_B20D098EAB6AEC49B0D8ABCA73900059" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_7A45380584A10ECE2E10ABCA7390E832" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TradeNamesMember" xlink:label="loc_us-gaap_TradeNamesMember_BCDAA3C998A5775E6A8EABCA73911D83" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_7A45380584A10ECE2E10ABCA7390E832" xlink:to="loc_us-gaap_TradeNamesMember_BCDAA3C998A5775E6A8EABCA73911D83" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_OtherIntangibleAssetsNotSubjectToAmortizationMember" xlink:label="loc_dgx_OtherIntangibleAssetsNotSubjectToAmortizationMember_31B9A855EBD7A788BB0AABCA73910B58" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_7A45380584A10ECE2E10ABCA7390E832" xlink:to="loc_dgx_OtherIntangibleAssetsNotSubjectToAmortizationMember_31B9A855EBD7A788BB0AABCA73910B58" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisposalGroupClassificationAxis" xlink:label="loc_us-gaap_DisposalGroupClassificationAxis_C5A0D449591B7F37684DABCA73911622" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsTable_8DF9D67EC3B80624AF62ABCA738E3AEE" xlink:to="loc_us-gaap_DisposalGroupClassificationAxis_C5A0D449591B7F37684DABCA73911622" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisposalGroupClassificationDomain" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_F461615AD80C322024D3ABCA73915BF1_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_C5A0D449591B7F37684DABCA73911622" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_F461615AD80C322024D3ABCA73915BF1_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisposalGroupClassificationDomain" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_F461615AD80C322024D3ABCA73915BF1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_C5A0D449591B7F37684DABCA73911622" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_F461615AD80C322024D3ABCA73915BF1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisposalGroupHeldforsaleNotDiscontinuedOperationsMember" xlink:label="loc_us-gaap_DisposalGroupHeldforsaleNotDiscontinuedOperationsMember_5992BF75627AF1E4431FABCA739154AA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_F461615AD80C322024D3ABCA73915BF1" xlink:to="loc_us-gaap_DisposalGroupHeldforsaleNotDiscontinuedOperationsMember_5992BF75627AF1E4431FABCA739154AA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_Goodwill" xlink:label="loc_us-gaap_Goodwill_5C674AB0E30F8C659277ABCA7391CEAD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_us-gaap_Goodwill_5C674AB0E30F8C659277ABCA7391CEAD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GoodwillAcquiredDuringPeriod" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_BAF0204E591347E51B63ABCA7391AE7D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_BAF0204E591347E51B63ABCA7391AE7D" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_Goodwillreclassifiedtocurrentandnoncurrentassetsheldforsale" xlink:label="loc_dgx_Goodwillreclassifiedtocurrentandnoncurrentassetsheldforsale_D3748C1EFF739E04E7A9ABEA52559DE4" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_dgx_Goodwillreclassifiedtocurrentandnoncurrentassetsheldforsale_D3748C1EFF739E04E7A9ABEA52559DE4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_2F836444D4B7CCB32520ABCA73919585" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_2F836444D4B7CCB32520ABCA73919585" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_0EB80F745ACBD3920170ABCA73910C56" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_0EB80F745ACBD3920170ABCA73910C56" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IntangibleAssetsGrossExcludingGoodwill" xlink:label="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_AD4AC71D56E4A3FAD6EDABCA73913845" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_AD4AC71D56E4A3FAD6EDABCA73913845" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_3043C2B304E377420AB9ABCA7391A7A3" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_3043C2B304E377420AB9ABCA7391A7A3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_B775F6CF06F2B9D5E644ABCA7392038F" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_B775F6CF06F2B9D5E644ABCA7392038F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AmortizationOfIntangibleAssets" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_5CB800C06D4E10AD4F25ABCA739203E0" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_5CB800C06D4E10AD4F25ABCA739203E0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseRemainderOfFiscalYear" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseRemainderOfFiscalYear_0E4C8DE9178F3FA282AFABCA739203E4" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseRemainderOfFiscalYear_0E4C8DE9178F3FA282AFABCA739203E4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths_F091E5772910652ACE0BABCA73925296" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths_F091E5772910652ACE0BABCA73925296" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo_501CD932D31635AAEE68ABCA73924BD1" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo_501CD932D31635AAEE68ABCA73924BD1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree_A5F49802593F52FCFE07ABCA7392EDE9" xlink:type="locator" />
    <definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree_A5F49802593F52FCFE07ABCA7392EDE9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour_4AE341B47BF27FE72888ABCA73928094" xlink:type="locator" />
    <definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour_4AE341B47BF27FE72888ABCA73928094" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive_DB34BE29C2AD9D4692C0ABCA73922BD8" xlink:type="locator" />
    <definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive_DB34BE29C2AD9D4692C0ABCA73922BD8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive_AB57097C31C7FF656B98ABCA73926B4F" xlink:type="locator" />
    <definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive_AB57097C31C7FF656B98ABCA73926B4F" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_FiniteLivedIntangibleAssetsEstimatedAmortizationExpenseTotal" xlink:label="loc_dgx_FiniteLivedIntangibleAssetsEstimatedAmortizationExpenseTotal_AB89A6525991F40F7635ABCA739262CB" xlink:type="locator" />
    <definitionArc order="18" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems_3A16DC63AF711E946F68ABCA73916FB0" xlink:to="loc_dgx_FiniteLivedIntangibleAssetsEstimatedAmortizationExpenseTotal_AB89A6525991F40F7635ABCA739262CB" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/GoodwillAndIntangibleAssetsTables" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/RelatedParties" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/RelatedPartiesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_1EE89C011C5CB98548D2AE8452BA1B96" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_1A5745C99AB0FC6AA689AE8452BA896D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_1EE89C011C5CB98548D2AE8452BA1B96" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_1A5745C99AB0FC6AA689AE8452BA896D" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_3C976C5E1B48AEA6C04FAE8452BA6A9C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_1A5745C99AB0FC6AA689AE8452BA896D" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_3C976C5E1B48AEA6C04FAE8452BA6A9C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="loc_us-gaap_RelatedPartyDomain_AA849805A096FC3AAEC6AE8452BA6DC9_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_3C976C5E1B48AEA6C04FAE8452BA6A9C" xlink:to="loc_us-gaap_RelatedPartyDomain_AA849805A096FC3AAEC6AE8452BA6DC9_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="loc_us-gaap_RelatedPartyDomain_AA849805A096FC3AAEC6AE8452BA6DC9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_3C976C5E1B48AEA6C04FAE8452BA6A9C" xlink:to="loc_us-gaap_RelatedPartyDomain_AA849805A096FC3AAEC6AE8452BA6DC9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityMethodInvesteeMember" xlink:label="loc_us-gaap_EquityMethodInvesteeMember_58BC132EE98C0CFA349AAE8452BA44AF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_AA849805A096FC3AAEC6AE8452BA6DC9" xlink:to="loc_us-gaap_EquityMethodInvesteeMember_58BC132EE98C0CFA349AAE8452BA44AF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_1EE1A17A17A291B22D12AE8452BAF5B8" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_1A5745C99AB0FC6AA689AE8452BA896D" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_1EE1A17A17A291B22D12AE8452BAF5B8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_7912B1508C9C4FA4B095AE8452BA95B6_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_1EE1A17A17A291B22D12AE8452BAF5B8" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_7912B1508C9C4FA4B095AE8452BA95B6_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_7912B1508C9C4FA4B095AE8452BA95B6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_1EE1A17A17A291B22D12AE8452BAF5B8" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_7912B1508C9C4FA4B095AE8452BA95B6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_7042C0581B364AF41D45AE8452BA45CD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_7912B1508C9C4FA4B095AE8452BA95B6" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_7042C0581B364AF41D45AE8452BA45CD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_258A1619410C45669EA9AE8452BA51A2" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_1A5745C99AB0FC6AA689AE8452BA896D" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_258A1619410C45669EA9AE8452BA51A2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_B19867F7D9028E34A634AE8452BA644F_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_258A1619410C45669EA9AE8452BA51A2" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_B19867F7D9028E34A634AE8452BA644F_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_B19867F7D9028E34A634AE8452BA644F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_258A1619410C45669EA9AE8452BA51A2" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_B19867F7D9028E34A634AE8452BA644F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember" xlink:label="loc_us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember_A75C35B4B46CC88C1577AE8452BA2ADF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_B19867F7D9028E34A634AE8452BA644F" xlink:to="loc_us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember_A75C35B4B46CC88C1577AE8452BA2ADF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RevenueFromRelatedParties" xlink:label="loc_us-gaap_RevenueFromRelatedParties_AE558A0EE1F7479AD7F1AE8452BAD829" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_1EE89C011C5CB98548D2AE8452BA1B96" xlink:to="loc_us-gaap_RevenueFromRelatedParties_AE558A0EE1F7479AD7F1AE8452BAD829" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccountsReceivableRelatedParties" xlink:label="loc_us-gaap_AccountsReceivableRelatedParties_BA6F080E66339634F114AE8452BA7595" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_1EE89C011C5CB98548D2AE8452BA1B96" xlink:to="loc_us-gaap_AccountsReceivableRelatedParties_BA6F080E66339634F114AE8452BA7595" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RelatedPartyTransactionOtherRevenuesFromTransactionsWithRelatedParty" xlink:label="loc_us-gaap_RelatedPartyTransactionOtherRevenuesFromTransactionsWithRelatedParty_477429A40016E9BDED4AAEA67AE125CB" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_1EE89C011C5CB98548D2AE8452BA1B96" xlink:to="loc_us-gaap_RelatedPartyTransactionOtherRevenuesFromTransactionsWithRelatedParty_477429A40016E9BDED4AAEA67AE125CB" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_OtherAccountsReceivableRelatedParties" xlink:label="loc_dgx_OtherAccountsReceivableRelatedParties_DD0D472FA3EAE5DB46B6AE8452BB922A" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_1EE89C011C5CB98548D2AE8452BA1B96" xlink:to="loc_dgx_OtherAccountsReceivableRelatedParties_DD0D472FA3EAE5DB46B6AE8452BB922A" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/RestructuringActivities" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/RestructuringActivitiesActivitiesOfRestructuringLiabilitiesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCostAndReserveLineItems" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_D46D5250D8F96AC5A0D628FD578643B8" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_78EBE00E1E5C18984D5028FD578600D8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_D46D5250D8F96AC5A0D628FD578643B8" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_78EBE00E1E5C18984D5028FD578600D8" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringPlanAxis" xlink:label="loc_us-gaap_RestructuringPlanAxis_F2C21246E012248D500E28FD57862ED0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_78EBE00E1E5C18984D5028FD578600D8" xlink:to="loc_us-gaap_RestructuringPlanAxis_F2C21246E012248D500E28FD57862ED0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringPlanDomain" xlink:label="loc_us-gaap_RestructuringPlanDomain_5E696265489BB6070B3128FD57863BDF_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_F2C21246E012248D500E28FD57862ED0" xlink:to="loc_us-gaap_RestructuringPlanDomain_5E696265489BB6070B3128FD57863BDF_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringPlanDomain" xlink:label="loc_us-gaap_RestructuringPlanDomain_5E696265489BB6070B3128FD57863BDF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_F2C21246E012248D500E28FD57862ED0" xlink:to="loc_us-gaap_RestructuringPlanDomain_5E696265489BB6070B3128FD57863BDF" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_InvigorateProgramMember" xlink:label="loc_dgx_InvigorateProgramMember_C023420DFDBF6174C73C28FD57867A8B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_5E696265489BB6070B3128FD57863BDF" xlink:to="loc_dgx_InvigorateProgramMember_C023420DFDBF6174C73C28FD57867A8B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCostAndReserveAxis" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_1417728DC4222378D2F628FD57867D0A" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_78EBE00E1E5C18984D5028FD578600D8" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_1417728DC4222378D2F628FD57867D0A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_89A0484077F8853AB3F928FD578606A9_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_1417728DC4222378D2F628FD57867D0A" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_89A0484077F8853AB3F928FD578606A9_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_89A0484077F8853AB3F928FD578606A9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_1417728DC4222378D2F628FD57867D0A" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_89A0484077F8853AB3F928FD578606A9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeSeveranceMember" xlink:label="loc_us-gaap_EmployeeSeveranceMember_1AA689A8B688016B12BD28FD57866B1E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_89A0484077F8853AB3F928FD578606A9" xlink:to="loc_us-gaap_EmployeeSeveranceMember_1AA689A8B688016B12BD28FD57866B1E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FacilityClosingMember" xlink:label="loc_us-gaap_FacilityClosingMember_C38EFEE237D7DD64BB8F28FD5786D5F3" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_89A0484077F8853AB3F928FD578606A9" xlink:to="loc_us-gaap_FacilityClosingMember_C38EFEE237D7DD64BB8F28FD5786D5F3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringReserveRollForward" xlink:label="loc_us-gaap_RestructuringReserveRollForward_B04BCBF3B5D7849FD9D228FD5786A668" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_D46D5250D8F96AC5A0D628FD578643B8" xlink:to="loc_us-gaap_RestructuringReserveRollForward_B04BCBF3B5D7849FD9D228FD5786A668" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringReserve" xlink:label="loc_us-gaap_RestructuringReserve_7699398CF20AE6F615AC28FD578683CD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_B04BCBF3B5D7849FD9D228FD5786A668" xlink:to="loc_us-gaap_RestructuringReserve_7699398CF20AE6F615AC28FD578683CD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCharges" xlink:label="loc_us-gaap_RestructuringCharges_2DA8E4CF8D371C22ED3028FD578639A9" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_B04BCBF3B5D7849FD9D228FD5786A668" xlink:to="loc_us-gaap_RestructuringCharges_2DA8E4CF8D371C22ED3028FD578639A9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PaymentsForRestructuring" xlink:label="loc_us-gaap_PaymentsForRestructuring_470601C6530CCFB9C67828FD578691AE" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_B04BCBF3B5D7849FD9D228FD5786A668" xlink:to="loc_us-gaap_PaymentsForRestructuring_470601C6530CCFB9C67828FD578691AE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringReserveAccrualAdjustment" xlink:label="loc_us-gaap_RestructuringReserveAccrualAdjustment_91FEA2F1DEC0619B5A7228FD5786F73F" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_B04BCBF3B5D7849FD9D228FD5786A668" xlink:to="loc_us-gaap_RestructuringReserveAccrualAdjustment_91FEA2F1DEC0619B5A7228FD5786F73F" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/RestructuringActivitiesNarrativeDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCostAndReserveLineItems" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_1C258DD7F0358CC7A4A528FD563F9BD6" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_C71052B2089571910E2928FD563FAA4D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_1C258DD7F0358CC7A4A528FD563F9BD6" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_C71052B2089571910E2928FD563FAA4D" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="loc_us-gaap_StatementScenarioAxis_2AA055D47B35E50BA9CB28FD563F565E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_C71052B2089571910E2928FD563FAA4D" xlink:to="loc_us-gaap_StatementScenarioAxis_2AA055D47B35E50BA9CB28FD563F565E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_BCCFC4BEE23B65DD270B28FD563FF8E9_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementScenarioAxis_2AA055D47B35E50BA9CB28FD563F565E" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_BCCFC4BEE23B65DD270B28FD563FF8E9_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_BCCFC4BEE23B65DD270B28FD563FF8E9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementScenarioAxis_2AA055D47B35E50BA9CB28FD563F565E" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_BCCFC4BEE23B65DD270B28FD563FF8E9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioForecastMember" xlink:label="loc_us-gaap_ScenarioForecastMember_34134D0EF1A90C21194928FD563FA4A1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScenarioUnspecifiedDomain_BCCFC4BEE23B65DD270B28FD563FF8E9" xlink:to="loc_us-gaap_ScenarioForecastMember_34134D0EF1A90C21194928FD563FA4A1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringPlanAxis" xlink:label="loc_us-gaap_RestructuringPlanAxis_9D10B37F135670FDB52A28FD563F4BDA" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_C71052B2089571910E2928FD563FAA4D" xlink:to="loc_us-gaap_RestructuringPlanAxis_9D10B37F135670FDB52A28FD563F4BDA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringPlanDomain" xlink:label="loc_us-gaap_RestructuringPlanDomain_B0805B68E11CCC3A973028FD563F48A9_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_9D10B37F135670FDB52A28FD563F4BDA" xlink:to="loc_us-gaap_RestructuringPlanDomain_B0805B68E11CCC3A973028FD563F48A9_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringPlanDomain" xlink:label="loc_us-gaap_RestructuringPlanDomain_B0805B68E11CCC3A973028FD563F48A9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_9D10B37F135670FDB52A28FD563F4BDA" xlink:to="loc_us-gaap_RestructuringPlanDomain_B0805B68E11CCC3A973028FD563F48A9" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_InvigorateProgramMember" xlink:label="loc_dgx_InvigorateProgramMember_C203F2752FC18370E5F428FD563F9DFD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_B0805B68E11CCC3A973028FD563F48A9" xlink:to="loc_dgx_InvigorateProgramMember_C203F2752FC18370E5F428FD563F9DFD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_7F1AD93245E60889D86E28FD563F9404" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_C71052B2089571910E2928FD563FAA4D" xlink:to="loc_us-gaap_RangeAxis_7F1AD93245E60889D86E28FD563F9404" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_0898D59C9953BD79485928FD563F4DBC_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_7F1AD93245E60889D86E28FD563F9404" xlink:to="loc_us-gaap_RangeMember_0898D59C9953BD79485928FD563F4DBC_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_0898D59C9953BD79485928FD563F4DBC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_7F1AD93245E60889D86E28FD563F9404" xlink:to="loc_us-gaap_RangeMember_0898D59C9953BD79485928FD563F4DBC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_B57710FA1667BDF476E628FD563F4645" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_0898D59C9953BD79485928FD563F4DBC" xlink:to="loc_us-gaap_MinimumMember_B57710FA1667BDF476E628FD563F4645" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_49B0E78AACECC372DFFB28FD563F1660" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_0898D59C9953BD79485928FD563F4DBC" xlink:to="loc_us-gaap_MaximumMember_49B0E78AACECC372DFFB28FD563F1660" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCostAndReserveAxis" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_9AA057454E511D37BE4B28FD563F1E63" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_C71052B2089571910E2928FD563FAA4D" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_9AA057454E511D37BE4B28FD563F1E63" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_D4CC4FDC911A7BCDBBAD28FD563F27F2_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_9AA057454E511D37BE4B28FD563F1E63" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_D4CC4FDC911A7BCDBBAD28FD563F27F2_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_D4CC4FDC911A7BCDBBAD28FD563F27F2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_9AA057454E511D37BE4B28FD563F1E63" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_D4CC4FDC911A7BCDBBAD28FD563F27F2" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_CostOfServicesMember" xlink:label="loc_dgx_CostOfServicesMember_9C8DB8CEC3458A746F3928FD563FB73F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_D4CC4FDC911A7BCDBBAD28FD563F27F2" xlink:to="loc_dgx_CostOfServicesMember_9C8DB8CEC3458A746F3928FD563FB73F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_F97D49D2B5988AD448FC28FD563FA5CF" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_D4CC4FDC911A7BCDBBAD28FD563F27F2" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_F97D49D2B5988AD448FC28FD563FA5CF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SeveranceCosts1" xlink:label="loc_us-gaap_SeveranceCosts1_F88DE489BDDCBD9A9A7228FD563FBAE5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_1C258DD7F0358CC7A4A528FD563F9BD6" xlink:to="loc_us-gaap_SeveranceCosts1_F88DE489BDDCBD9A9A7228FD563FBAE5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherRestructuringCosts" xlink:label="loc_us-gaap_OtherRestructuringCosts_3EB915C654ABFE96F5D028FD563FC010" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_1C258DD7F0358CC7A4A528FD563F9BD6" xlink:to="loc_us-gaap_OtherRestructuringCosts_3EB915C654ABFE96F5D028FD563FC010" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCharges" xlink:label="loc_us-gaap_RestructuringCharges_2A95C602E6B9DAA27CC228FD563F4629" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_1C258DD7F0358CC7A4A528FD563F9BD6" xlink:to="loc_us-gaap_RestructuringCharges_2A95C602E6B9DAA27CC228FD563F4629" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/RestructuringActivitiesPreTaxRestructuringAndIntegrationChargesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCostAndReserveLineItems" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_48D588D6CE2CA6D722B728FD59D7BF43" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_49D33362D64E9D9E4AC128FD59D715E7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_48D588D6CE2CA6D722B728FD59D7BF43" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_49D33362D64E9D9E4AC128FD59D715E7" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringPlanAxis" xlink:label="loc_us-gaap_RestructuringPlanAxis_0B4A6AB0B558FBB2537128FD59D7D6F5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_49D33362D64E9D9E4AC128FD59D715E7" xlink:to="loc_us-gaap_RestructuringPlanAxis_0B4A6AB0B558FBB2537128FD59D7D6F5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringPlanDomain" xlink:label="loc_us-gaap_RestructuringPlanDomain_217A6695BB4566A282CF28FD59D70868_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_0B4A6AB0B558FBB2537128FD59D7D6F5" xlink:to="loc_us-gaap_RestructuringPlanDomain_217A6695BB4566A282CF28FD59D70868_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringPlanDomain" xlink:label="loc_us-gaap_RestructuringPlanDomain_217A6695BB4566A282CF28FD59D70868" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_0B4A6AB0B558FBB2537128FD59D7D6F5" xlink:to="loc_us-gaap_RestructuringPlanDomain_217A6695BB4566A282CF28FD59D70868" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_InvigorateProgramMember" xlink:label="loc_dgx_InvigorateProgramMember_A93BBAFDD2C78543211828FD59D7B2F8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_217A6695BB4566A282CF28FD59D70868" xlink:to="loc_dgx_InvigorateProgramMember_A93BBAFDD2C78543211828FD59D7B2F8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SeveranceCosts1" xlink:label="loc_us-gaap_SeveranceCosts1_B14B386B4515D32ED82828FD59D7B343" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_48D588D6CE2CA6D722B728FD59D7BF43" xlink:to="loc_us-gaap_SeveranceCosts1_B14B386B4515D32ED82828FD59D7B343" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherRestructuringCosts" xlink:label="loc_us-gaap_OtherRestructuringCosts_F416D9A4D198C09A650A28FD59D7C53F" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_48D588D6CE2CA6D722B728FD59D7BF43" xlink:to="loc_us-gaap_OtherRestructuringCosts_F416D9A4D198C09A650A28FD59D7C53F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCostsAndAssetImpairmentCharges" xlink:label="loc_us-gaap_RestructuringCostsAndAssetImpairmentCharges_77A50A8E1E97AC8A97EA28FD59D78FE8" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_48D588D6CE2CA6D722B728FD59D7BF43" xlink:to="loc_us-gaap_RestructuringCostsAndAssetImpairmentCharges_77A50A8E1E97AC8A97EA28FD59D78FE8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCharges" xlink:label="loc_us-gaap_RestructuringCharges_974B08F86E4698FA028428FD59D78C94" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_48D588D6CE2CA6D722B728FD59D7BF43" xlink:to="loc_us-gaap_RestructuringCharges_974B08F86E4698FA028428FD59D78C94" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/RestructuringActivitiesTables" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/StockholdersEquityAndRedeemableNoncontrollingInterest" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/StockholdersEquityAndRedeemableNoncontrollingInterestNarrativeDetails" xlink:type="extended">
    <loc xlink:href="dgx-20150930.xsd#dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestLineItems" xlink:label="loc_dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestLineItems_7559E9697B98FBA07774AF567A8357F7" xlink:type="locator" />
    <loc xlink:href="dgx-20150930.xsd#dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestTable" xlink:label="loc_dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestTable_59BE435A5A4B55E68235AF567A81347A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestLineItems_7559E9697B98FBA07774AF567A8357F7" xlink:to="loc_dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestTable_59BE435A5A4B55E68235AF567A81347A" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareRepurchaseProgramAxis" xlink:label="loc_us-gaap_ShareRepurchaseProgramAxis_82D14BA6A20CE3C83312AF31C79EE4B5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestTable_59BE435A5A4B55E68235AF567A81347A" xlink:to="loc_us-gaap_ShareRepurchaseProgramAxis_82D14BA6A20CE3C83312AF31C79EE4B5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_3FA161C01C87501BD23AAF31C79EA127_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_82D14BA6A20CE3C83312AF31C79EE4B5" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_3FA161C01C87501BD23AAF31C79EA127_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_3FA161C01C87501BD23AAF31C79EA127" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_82D14BA6A20CE3C83312AF31C79EE4B5" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_3FA161C01C87501BD23AAF31C79EA127" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_ShareRepurchasesOpenMarketMember" xlink:label="loc_dgx_ShareRepurchasesOpenMarketMember_9AB0DA289FF4E3AD9D19AF31C79E1A4C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareRepurchaseProgramDomain_3FA161C01C87501BD23AAF31C79EA127" xlink:to="loc_dgx_ShareRepurchasesOpenMarketMember_9AB0DA289FF4E3AD9D19AF31C79E1A4C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_0FE2FFBB496AC4601B70AF31C79E54E3" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestTable_59BE435A5A4B55E68235AF567A81347A" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_0FE2FFBB496AC4601B70AF31C79E54E3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_96B5596A417AEDFE80C7AF31C79EA20B_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0FE2FFBB496AC4601B70AF31C79E54E3" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_96B5596A417AEDFE80C7AF31C79EA20B_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_96B5596A417AEDFE80C7AF31C79EA20B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0FE2FFBB496AC4601B70AF31C79E54E3" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_96B5596A417AEDFE80C7AF31C79EA20B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SubsidiariesMember" xlink:label="loc_us-gaap_SubsidiariesMember_A36977FDD7BDEB36EC5DAF31C79EDC21" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_96B5596A417AEDFE80C7AF31C79EA20B" xlink:to="loc_us-gaap_SubsidiariesMember_A36977FDD7BDEB36EC5DAF31C79EDC21" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_CAF87FCFAD3BD06880A2AF31C79ED32C" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestTable_59BE435A5A4B55E68235AF567A81347A" xlink:to="loc_dei_LegalEntityAxis_CAF87FCFAD3BD06880A2AF31C79ED32C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_B8D02D6D0EA243A29DE7AF31C79F28DE_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_CAF87FCFAD3BD06880A2AF31C79ED32C" xlink:to="loc_dei_EntityDomain_B8D02D6D0EA243A29DE7AF31C79F28DE_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_B8D02D6D0EA243A29DE7AF31C79F28DE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_CAF87FCFAD3BD06880A2AF31C79ED32C" xlink:to="loc_dei_EntityDomain_B8D02D6D0EA243A29DE7AF31C79F28DE" xlink:type="arc" />
    <loc xlink:href="dgx-20150930.xsd#dgx_UMassMember" xlink:label="loc_dgx_UMassMember_59989B5B5F474575DBCDAF31C79FF543" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_B8D02D6D0EA243A29DE7AF31C79F28DE" xlink:to="loc_dgx_UMassMember_59989B5B5F474575DBCDAF31C79FF543" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommonStockDividendsPerShareDeclared" xlink:label="loc_us-gaap_CommonStockDividendsPerShareDeclared_643547F3800CFEB08A6AAF31C79F8016" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestLineItems_7559E9697B98FBA07774AF567A8357F7" xlink:to="loc_us-gaap_CommonStockDividendsPerShareDeclared_643547F3800CFEB08A6AAF31C79F8016" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1" xlink:label="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_741E3D12D864FAA05291AF31C79F4D70" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestLineItems_7559E9697B98FBA07774AF567A8357F7" xlink:to="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_741E3D12D864FAA05291AF31C79F4D70" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TreasuryStockSharesAcquired" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_6B370FD0B2ED8221D89DAF31C79F9A79" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestLineItems_7559E9697B98FBA07774AF567A8357F7" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_6B370FD0B2ED8221D89DAF31C79F9A79" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TreasuryStockAcquiredAverageCostPerShare" xlink:label="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_BD4053C611B5DD8C6BE5AF31C79FE1CD" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestLineItems_7559E9697B98FBA07774AF567A8357F7" xlink:to="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_BD4053C611B5DD8C6BE5AF31C79FE1CD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_A09439F9677C052C494EAF31C79FC8DE" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestLineItems_7559E9697B98FBA07774AF567A8357F7" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_A09439F9677C052C494EAF31C79FC8DE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesTreasuryStockReissued" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesTreasuryStockReissued_4BD688E23823043B57B2AF31C79FD3F5" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestLineItems_7559E9697B98FBA07774AF567A8357F7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesTreasuryStockReissued_4BD688E23823043B57B2AF31C79FD3F5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners" xlink:label="loc_us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners_C88659B816CAC464FEC4AF57F3DAB0DB" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestLineItems_7559E9697B98FBA07774AF567A8357F7" xlink:to="loc_us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners_C88659B816CAC464FEC4AF57F3DAB0DB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount" xlink:label="loc_us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount_9AD695183F8491EA3CBAAF31C79F7F22" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dgx_ScheduleofStockholdersEquityandRedeemableNoncontrollingInterestLineItems_7559E9697B98FBA07774AF567A8357F7" xlink:to="loc_us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount_9AD695183F8491EA3CBAAF31C79F7F22" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://questdiagnostics.com/role/SummaryOfSignificantAccountingPolicies" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/SummaryOfSignificantAccountingPoliciesDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/SummaryOfSignificantAccountingPoliciesPolicies" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/SupplementalCashFlowOtherData" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/SupplementalCashFlowOtherDataDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://questdiagnostics.com/role/SupplementalCashFlowOtherDataTables" xlink:type="extended" />
</linkbase>
