<PAGE>

                                                                    EXHIBIT 99.7

                               LETTER TO CLIENTS

                               OFFER TO EXCHANGE
                       0.3256 OF A SHARE OF COMMON STOCK
          (INCLUDING THE ASSOCIATED RIGHT TO PURCHASE PREFERRED STOCK)

                                       OF

                         QUEST DIAGNOSTICS INCORPORATED

                                       OR

                                 $26.50 IN CASH

                                      FOR

                     EACH OUTSTANDING SHARE OF COMMON STOCK

                                       OF

                               UNILAB CORPORATION

SUBJECT, IN EACH CASE, TO THE PRORATION AND ELECTION PROCEDURES DESCRIBED IN THE
                                   PROSPECTUS
              AND THE RELATED LETTER OF ELECTION AND TRANSMITTAL.

  THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00 MIDNIGHT, NEW YORK CITY
 TIME, ON MONDAY, JUNE 17, 2002, UNLESS THE OFFER IS EXTENDED. SHARES TENDERED
 PURSUANT TO THIS OFFER MAY BE WITHDRAWN AT ANY TIME PRIOR TO THE EXPIRATION OF
                                   THE OFFER.

                                                                    May 15, 2002

To Our Clients:

     Enclosed for your consideration is the Prospectus, dated May 15, 2002 (the
"Prospectus"), and the related Letter of Election and Transmittal (which,
together with the Prospectus and any amendments or supplements thereto,
collectively constitute the "Offer") in connection with the offer by Quest
Diagnostics Newco Incorporated, a Delaware corporation ("Purchaser") and a
wholly owned subsidiary of Quest Diagnostics Incorporated, a Delaware
corporation ("Quest Diagnostics"), to exchange each outstanding share of common
stock, par value $.01 per share (the "Shares"), of Unilab Corporation, a
Delaware corporation (the "Company"), for (i) 0.3256 of a share of common stock,
par value $.01 per share, of Quest Diagnostics ("Quest Diagnostics Shares"), or
(ii) $26.50 in cash, without interest, at the election of the holder thereof and
upon the terms and subject to the conditions set forth in the Prospectus and in
the Letter of Election and Transmittal.

     WE ARE THE HOLDER OF RECORD (DIRECTLY OR INDIRECTLY) OF SHARES HELD FOR
YOUR ACCOUNT. A TENDER OF SUCH SHARES CAN BE MADE ONLY BY US AS THE HOLDER OF
RECORD AND PURSUANT TO YOUR INSTRUCTIONS. THE ENCLOSED LETTER OF ELECTION AND
TRANSMITTAL IS FURNISHED TO YOU FOR YOUR INFORMATION ONLY AND CANNOT BE USED BY
YOU TO TENDER SHARES HELD BY US FOR YOUR ACCOUNT.

     We request instructions as to whether you wish to have us tender on your
behalf any or all of the Shares held by us for your account, upon the terms and
subject to the conditions set forth in the Prospectus and the Letter of Election
and Transmittal, and, if you wish to tender, the consideration that you wish to
receive in exchange for your Shares.
<PAGE>

     Your attention is invited to the following:

          1.  Purchaser is offering to exchange each outstanding Share for (i)
     0.3256 of a Quest Diagnostics Share or (ii) $26.50 in cash, without
     interest, at your election and subject to the proration procedures
     described in the Prospectus and the Letter of Election and Transmittal.

          2.  The aggregate cash consideration that holders of Shares may
     receive in the Offer is subject to a pro rata reduction because not more
     than 30% of the Shares outstanding immediately prior to the expiration of
     the Offer can be exchanged for cash. An election to receive Quest
     Diagnostics shares for all of your Shares is not subject to proration.

          3.  If you instruct us to tender Shares but fail to make a proper
     election, you will be deemed to have elected to receive Quest Diagnostics
     Shares in exchange for your Shares, and upon completion of the Offer you
     will receive 0.3256 of a Quest Diagnostics Share for each Share you validly
     tender for exchange.

          4.  The Offer is being made for all outstanding Shares.

          5.  The Offer is being made pursuant to the Agreement and Plan of
     Merger, dated as of April 2, 2002, and amended as of May 13, 2002 (as
     amended, the "Merger Agreement"), among Quest Diagnostics, Purchaser and
     the Company. The Merger Agreement provides, among other things, that
     following completion of the Offer and the satisfaction or, if permissible,
     waiver of all conditions set forth in the Merger Agreement, and in
     accordance with the Delaware General Corporation Law ("Delaware Law"), the
     Company will be merged with and into Purchaser or, depending on certain tax
     matters, Purchaser will be merged with and into the Company (the "Merger").
     At the effective time of the Merger, each outstanding Share (other than
     Shares held in the Company's treasury or Shares held by Quest Diagnostics,
     Purchaser or any wholly owned subsidiary of Quest Diagnostics or Purchaser,
     or Shares held by any stockholder that has perfected appraisal rights, if
     available) will be cancelled and converted into 0.3256 of a Quest
     Diagnostics Share, upon the terms and conditions of the Merger Agreement.
     Notwithstanding the amount, if any, of cash paid in the offer, stockholders
     who do not tender Shares in the Offer will not receive any cash
     consideration in the Merger (except for cash, if any, that is paid in lieu
     of fractional Quest Diagnostics Shares or following the exercise of
     appraisal rights, if applicable). The Merger Agreement is more fully
     described in the section of the Prospectus entitled "The Merger Agreement".

          6.  The Board of Directors of the Company has determined that the
     Merger Agreement and the transactions contemplated thereby, including each
     of the Offer and the Merger, are fair to, and in the best interests of, the
     Company and the holders of Shares, has approved and declared advisable the
     Merger Agreement and the transactions contemplated thereby, including each
     of the Offer and the Merger, and recommends that holders of Shares accept
     the Offer and tender their Shares pursuant to the Offer.

          7.  THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00 MIDNIGHT, NEW
     YORK CITY TIME, ON MONDAY, JUNE 17, 2002, UNLESS THE OFFER IS EXTENDED.

          8.  The Offer is conditioned upon, among other things, (i) there
     having been validly tendered and not withdrawn prior to the expiration of
     the Offer that number of Shares which constitutes at least 50.1% of the
     aggregate number of outstanding Shares on a fully diluted basis (as though
     all options exercisable for Shares had been converted, exercised or
     exchanged) and (ii) the applicable waiting period under the
     Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended, having
     expired or been terminated prior to the expiration of the Offer. The Offer
     is also subject to other conditions set forth in the Prospectus, which you
     should review in detail.

          9.  Any stock transfer taxes applicable to the sale of Shares to
     Purchaser pursuant to the Offer will be paid by the Company, except as
     otherwise provided in the Letter of Election and Transmittal.

     The Offer is being made solely by means of the Prospectus and the related
Letter of Election and Transmittal (and any supplements and amendments thereto)
and is being made to all holders of Shares. Except as disclosed in the
Prospectus, Quest Diagnostics and Purchaser are not aware of any state in which
the making of the Offer or the acceptance of Shares pursuant to the Offer is
prohibited by any administrative or judicial action pursuant to any valid state
statute. If Quest Diagnostics or Purchaser becomes aware of any valid state
statute prohibiting the making of the Offer or the acceptance of Shares pursuant
thereto, Quest Diagnostics and Purchaser will make a good faith effort to comply
with such state statute. If, after such good faith effort, Quest Diagnostics and
Purchaser cannot comply with such
                                        2
<PAGE>

state statute, the Offer will not be made to (nor will tenders be accepted from
or on behalf of) the holders of Shares residing in any such state. In any
jurisdiction in which the securities, blue sky or other laws require the Offer
to be made by a licensed broker or dealer, the Offer will be deemed to be made
on behalf of Quest Diagnostics and Purchaser by Merrill Lynch & Co., the Dealer
Manager for the Offer, or one or more registered brokers or dealers licensed
under the laws of such jurisdiction.

     If you wish to have us tender any or all of your Shares, please so instruct
us by completing, executing and returning to us the instruction form set forth
on the reverse side of this letter. An envelope to return your instruction form
to us is enclosed. If you authorize the tender of your Shares, all your Shares
will be tendered unless otherwise specified on the reverse side of this letter.
YOUR INSTRUCTIONS SHOULD BE FORWARDED TO US IN SUFFICIENT TIME TO PERMIT US TO
SUBMIT A TENDER ON YOUR BEHALF PRIOR TO THE EXPIRATION DATE OF THE OFFER.

                                        3
<PAGE>

                        INSTRUCTIONS WITH RESPECT TO THE
                               OFFER TO EXCHANGE
                       0.3256 OF A SHARE OF COMMON STOCK
          (INCLUDING THE ASSOCIATED RIGHT TO PURCHASE PREFERRED STOCK)

                                       OF

                         QUEST DIAGNOSTICS INCORPORATED

                                       OR

                                 $26.50 IN CASH

                                      FOR

                     EACH OUTSTANDING SHARE OF COMMON STOCK

                                       OF

                               UNILAB CORPORATION

SUBJECT, IN EACH CASE, TO THE PRORATION AND ELECTION PROCEDURES DESCRIBED IN THE
                                   PROSPECTUS
              AND THE RELATED LETTER OF ELECTION AND TRANSMITTAL.

     The undersigned acknowledge(s) receipt of your letter and the enclosed
Prospectus, dated May 15, 2002 (the "Prospectus"), and the related Letter of
Election and Transmittal in connection with the offer by Quest Diagnostics Newco
Incorporated, a Delaware corporation and a wholly owned subsidiary of Quest
Diagnostics Incorporated, a Delaware corporation ("Quest Diagnostics"), to
exchange each outstanding share of common stock, par value $.01 per share (the
"Shares"), of Unilab Corporation, a Delaware corporation, for (i) 0.3256 of a
share of common stock, par value $.01 per share, of Quest Diagnostics ("Quest
Diagnostics Shares"), or (ii) $26.50 in cash, without interest, at the election
of the undersigned, upon the terms and subject to the conditions set forth in
the Prospectus and the related Letter of Election and Transmittal, including the
terms relating to proration.

     This will instruct you to tender the number of Shares indicated below (or,
if no number is indicated below, all Shares) that are held by you for the
account of the undersigned, upon the terms and subject to the conditions set
forth in the Prospectus and the related Letter of Election and Transmittal. The
undersigned acknowledges that failure to properly make an election will result
in the undersigned being deemed to have elected to receive Quest Diagnostics
Shares, in which case, upon completion of the offer, the undersigned will
receive 0.3256 of a Quest Diagnostics Share for each Share validly tendered for
exchange.

[ ]  The undersigned is electing to receive cash in exchange for all the
     undersigned's Shares, subject to proration.

[ ]  The undersigned is electing to receive Quest Diagnostics Shares in exchange
     for all the undersigned's Shares.

[ ]  The undersigned is electing to exchange ____ Shares for cash and to
     exchange the remainder of the undersigned's Shares for Quest Diagnostics
     Shares.

                                        4
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<Table>
<S>                                                               <C>

  Account No.:                                                                             SIGN HERE
                                                                  -----------------------------------------------------------
  Number of Shares to Be Tendered:                                -----------------------------------------------------------
  ----------------------------------------------- (1)                                    SIGNATURE(S)
                                                                  -----------------------------------------------------------
  Dated: ---------------------------------------- , 2002          -----------------------------------------------------------
                                                                                 PLEASE TYPE OR PRINT NAME(S)
                                                                  -----------------------------------------------------------
                                                                  -----------------------------------------------------------
                                                                  -----------------------------------------------------------
                                                                                 PLEASE TYPE OR PRINT ADDRESS
                                                                  -----------------------------------------------------------
                                                                                AREA CODE AND TELEPHONE NUMBER
                                                                  -----------------------------------------------------------
                                                                       TAXPAYER IDENTIFICATION OR SOCIAL SECURITY NUMBER
</Table>

---------------

(1) Unless otherwise indicated, it will be assumed that all Shares held by us
    for your account are to be tendered.

                                        5

