
<PAGE>   1
                                                                   EXHIBIT 10.26




                               September 25, 1995



Quest Medical, Inc.
One Allentown Parkway
Allen, Texas  75002
Attention:  Mr. F. Robert Merrill III

Ladies and Gentlemen:

         In accordance with recent discussions, NationsBank of Texas, N.A.
("Lender") is pleased to make the following commitment to Quest Medical, Inc.
("Borrower").  THIS COMMITMENT LETTER SUPERSEDES AND REPLACES ANY COMMITMENT
LETTERS PREVIOUSLY DELIVERED TO BORROWER BY LENDER WITH RESPECT TO THE FACILITY
DESCRIBED BELOW, AND ANY RELATED ORAL OR WRITTEN COMMITMENTS OR OFFERS.

         Lender and Borrower previously entered into the First Amended and
Restated Credit Agreement dated as of March 31, 1995 (the "Existing Credit
Agreement"), pursuant to which Lender extended to Borrower a revolving credit
and term loan facility, subject to the terms and conditions set forth therein.
Subject to the terms of this agreement, Lender agrees to amend or restate the
Existing Credit Agreement to delete Facility B and add a new $15,000,000
facility, the proceeds of which will be used to make acquisitions described in
this agreement ("Acquisition Facility").  Facility A in the Existing Credit
Agreement will be retained, and amended or restated.

         The Acquisition Facility is contingent upon the occurrence of a public
offering of Borrower's common capital stock which is registered under the
Securities Act of 1933 and underwritten by a nationally recognized investment
banking firm resulting in net proceeds to Borrower of not less than
$17,000,000.00 (a "Qualified Offering").

         The Acquisition Facility, and the Existing Credit Agreement (as
amended or restated), shall include the basic terms and conditions set forth
below.  Capitalized terms used herein shall, unless otherwise indicated, have
the respective meanings set forth in the Existing Credit Agreement.

A.       ACQUISITION FACILITY

         1.      Commitment Amount.  The Acquisition Facility shall be in an
amount of up to $15,000,000.00 (the "Commitment").

         2.      Availability.  Subject to the conditions outlined below, the
Commitment shall be available in multiple advances on or after the closing date
and prior to maturity.
<PAGE>   2
Quest Medical, Inc.
September 25, 1995
Page 2


         3.      Use of Proceeds.  The Commitment shall be used by Borrower for
the acquisition of (a) all of the outstanding equity interest of any Person or
(b) all or a portion of the assets of any Person; provided, (i) the board of
directors of such Person has approved such acquisition and such approval has
not been revoked, and (ii) such Person or the assets of such Person the subject
of the proposed acquisition are used in the research, development, design,
production or marketing of Devices (as that term is defined in the Federal
Food, Drug and Cosmetic Act, 21 U.S.C.  Section  301, et seq.) (each, a
"Permitted Acquisition").

         4.      Interest Rate and Interest Payments.

                 (a)      Interest on advances under the Acquisition Facility
         shall accrue at the lesser of:  (i) at Borrower's option (A) the Prime
         Base Rate plus the Applicable Margin (defined below), or (B) the LIBOR
         Rate Basis plus the Applicable Margin; and (ii) the Highest Lawful
         Rate.  Past-due principal of, and to the extent permitted by
         Applicable Law, interest on, the Acquisition Facility shall bear
         interest at the Highest Lawful Rate.

                 (b)      The "Applicable Margin" means, at the time of
         determination thereof, the interest margin over the Prime Base Rate or
         the LIBOR Rate Basis, as the case may be, based on the Margin Ratio
         described below as follows:


<TABLE>
<CAPTION>
  =====================================================================================================
                                                     Applicable Margin
                                                Prime Base Rate Borrowings           Applicable Margin
                 Margin Ratio                                                         LIBOR Borrowings
  -----------------------------------------------------------------------------------------------------
  <S>                                                     <C>                              <C>
  Equal to or greater than 2.0 to 1.0                      .25%                             2.0%
  -----------------------------------------------------------------------------------------------------
  Equal to or greater than 1.5 to 1.0                       0%                             1.75%
  -----------------------------------------------------------------------------------------------------
  Less than 1.5 to 1.0                                      0%                             1.50%
  =====================================================================================================
</TABLE>

                 (c)      "Margin Ratio" means the ratio, as at any date of
         determination, of (i) the sum of the aggregate unpaid principal of all
         outstanding Advances, plus all accrued, unpaid interest on all
         Advances, plus all other Obligations to (ii) EBITDA.

                 (d)      Interest with respect to any portion of the
         Acquisition Facility subject to the Prime Base Rate shall be due and
         payable quarterly as it accrues.  Interest with respect to any portion
         of the Acquisition Facility subject to the LIBOR Rate Basis shall be
         payable at the end of each Interest Period, but not less frequently
         than quarterly.

         5.      Term.  The outstanding principal of, and any unpaid interest
on, the Acquisition Facility not evidenced by a Term Note (as defined below)
will be repaid at maturity (being the
<PAGE>   3
Quest Medical, Inc.
September 25, 1995
Page 3


second anniversary of the Qualified Offering).  The outstanding principal of,
and any unpaid interest on, the Acquisition Facility evidenced by a Term Note
will be repaid in equal quarterly installments, commencing with the first
quarterly date occurring after the date of such Term Note, with the final
payment due on the fifth anniversary of the date of such Term Note.

         6.      Commitment Fee.  Borrower shall pay to Lender a non-refundable
commitment fee equal to $50,000.00 upon execution of the definitive loan and
collateral documents.  In addition, Borrower shall pay to Lender, quarterly in
arrears, a commitment fee equal to one forth of one percent (1/4%) per annum
(calculated on the basis of actual number of days elapsed in a year consisting
of 360 days) on the unused portion of the Commitment.

         7.      Conditions to Advances.  In addition to the conditions
precedent described in Section D.1., the obligation of Lender to make advances
under the Acquisition Facility, is subject to the following additional
conditions:

                 (a)      Borrower shall have provided to Lender (i) current
         financial statements of and historical operating information on the
         Person which, or the assets of which, is the proposed Permitted
         Acquisition, (ii) a pro-forma balance sheet of Borrower, together with
         pro-forma operating projections of the three year period following the
         acquisition, (iii) pro forma financial statements which consolidate
         the financial statements of Borrower and the Person of which is the
         proposed Permitted Acquisition (or if assets, only, of such Person are
         to be acquired, financial statements based on the operations of such
         assets as a stand-alone entity), prepared as if Borrower and such
         Person (or its assets) had operated as a single entity during the
         twelve-month period preceding the effective date of proposed Permitted
         Acquisition, (iv) a pro-forma compliance certificate showing
         calculations of all financial covenants set forth in the loan
         agreement, based on the financial statements described inclause (iii),
         (v) a copy of the results of Borrower's due diligence with respect to
         the Person which, or the assets of which, is the proposed Permitted
         Acquisition and (vi) such other documents and information as Lender
         may reasonably request;

                 (b)      If the aggregate consideration payable by Borrower in
         respect of the specific Permitted Acquisition equals or exceeds
         $5,000,000, Lender shall have notified Borrower within ten days of
         receipt of the information described in Section A.7.(a) in form and
         substance acceptable to Lender that Lender has approved the Permitted
         Acquisition;

                 (c)      If the principal amount of any single proposed
         advance equals or exceeds $3,000,000, Borrower shall execute and
         deliver to Lender a promissory note (a "Term Note"), dated as of the
         date of the proposed advance.  If after giving effect to a proposed
         advance the principal amount of which is less than $3,000,000, the
         aggregate outstanding
<PAGE>   4
Quest Medical, Inc.
September 25, 1995
Page 4


         principal of all advances not evidenced by a Term Note equals or
         exceeds $3,000,000, Borrower shall execute and deliver to Lender a
         Term Note, dated as of the date of the proposed advance, in the
         principal amount of the sum of the proposed advance plus the principal
         amount of existing advances not evidenced by a Term Note; and

                 (d)      No Default or Event of Default exists, or will exist
         after giving effect to the Permitted Acquisition.

         8.      Other Terms.  The Commitment shall otherwise be generally
subject to the same basic terms and conditions set forth in the Existing Credit
Agreement.

B.       COLLATERAL

         The Acquisition Facility, if and when drawn upon, shall be secured by
the assets referred to in the Collateral Documents.

C.       CONDITIONS, REPRESENTATIONS, COVENANTS, AND EVENTS OF DEFAULT

         1.      Conditions Precedent.  Lender's obligations to amend or
restate the Existing Credit Agreement and to make the loans described above
shall be subject to conditions precedent that are standard in loan
documentation for similar loans made by Lender, including the conditions that:
(a) Lender has received loan and collateral documents in form and substance
satisfactory to Lender; (b) all representations and warranties made by Borrower
to Lender shall be true and correct as of the date of such amendment or
restatement and funding; (c) as of the date of such amendment or restatement
and funding, no material adverse change has occurred in the business or
financial condition of Borrower; and (d) as of the date of such amendment or
restatement and funding, there exists no Event of Default (or event with which
notice or lapse of time or both could constitute an Event of Default) under the
loan agreement.

         2.      Representations and Warranties.  The loan documentation will
include representations and warranties similar to those set forth in the
Existing Credit Agreement.

         3.      Affirmative Covenants.  The loan documentation will include
affirmative covenants similar to those set forth in the Existing Credit
Agreement.

         4.      Negative Covenants.  The loan documentation will include
negative covenants similar to those set forth in the Existing Credit Agreement,
which shall be applicable if and when the Acquisition Facility is drawn upon.
<PAGE>   5
Quest Medical, Inc.
September 25, 1995
Page 5


         5.      Financial Covenants.  The Existing Credit Agreement, as
amended or restated shall include the following financial covenants.  Each of
the following ratios shall be determined on a consolidated basis pursuant to
GAAP.

                 (a)      Fixed Charges Coverage Ratio.  Borrower shall not
         permit the Fixed Charges Coverage Ratio to be less than as indicated
         below, as at the end of each fiscal quarter ending during the period
         indicated:

                 Effective Date and thereafter                      2.50 to 1.00

                 (b)      Total Liabilities to Tangible Net Worth Ratio.
         Borrower shall not permit the ratio of Total Liabilities to Tangible
         Net Worth to be greater than as indicated below, as at the end of each
         fiscal quarter ending on the date or during the period indicated:

                 Effective Date through September 30, 1996          2.50 to 1.00
                 October 1, 1996 and thereafter                     1.75 to 1.00

                 (c)      Current Maturities Coverage Ratio.  Borrower shall
         not permit the Current Maturities Coverage Ratio to be less than as
         indicated below, as at the end of each fiscal quarter ending during
         the period indicated:

                 Effective Date and thereafter                      2.00 to 1.00

                 (d)      Margin Ratio.  Borrower shall not permit the ratio of
         (i) the sum of the aggregate unpaid principal of all outstanding
         Advances, plus all accrued, unpaid interest on all Advances, plus all
         other Obligations to (ii) EBITDA, to be greater than 2.50 to 1.00 as
         at the end of each fiscal quarter.

         6.      Defaults.  The loan documentation will include events of
default similar to the Events of Default set forth in the Existing Credit
Agreement.

D.       GENERAL PROVISIONS

         1.      Financing Documents.  The loan documents to be executed in
connection with the Acquisition Facility shall be in form and substance
mutually acceptable to Lender and Borrower.  Borrower shall deliver certified
resolutions, opinions of counsel, incumbency certificates, collateral documents
and related instruments, in form and substance reasonably satisfactory to
Lender and its counsel, as Lender shall require.

         2.      Expenses.  Borrower agrees to pay the reasonable costs,
expenses and fees (including reasonable attorney's fees) incurred or suffered
by Lender in connection with the
<PAGE>   6
Quest Medical, Inc.
September 25, 1995
Page 6


negotiation, preparation and documentation of the Acquisition Facility and the
amendment or restatement of the Existing Credit Agreement.  If the negotiations
concerning the Acquisition Facility and the amendment or restatement of the
Existing Credit Agreement are terminated prior to execution of final
documentation, Borrower shall pay the above- mentioned costs, expenses and fees
of Lender incurred prior to, or as a result of, such termination.

         3.      Reliance.  There are not third party beneficiaries of this
commitment letter, and no party other than Borrower shall be entitled to rely
on this commitment letter.  This commitment letter is not assignable.

         4.      Texas law.  This document shall be deemed to be executed and
performable in, and governed by the substantive laws of, the State of Texas.

         5.      Scope of Commitment.  The foregoing is intended to provide a
substantive outline of Lender's commitment rather than a complete statement of
all terms, conditions and documents which would be required in connection with
the Acquisition Facility and the amendment or restatement of the Existing
Credit Agreement.  It is possible that substantive terms or conditions may be
changed in order to account for or reflect changes in statutory or regulatory
authorities governing the subject matter of the transaction.

         6.      Increased Costs.  In addition, and without limiting the
generality of the foregoing, if any law, regulation or the interpretation
thereof by any court or administrative or governmental authority shall either
impose, modify or deem applicable any capital reserve, insurance premium, or
similar requirement against commitments or loans made by Lender and the result
thereof shall be to increase the cost to Lender of making, funding, or
maintaining this commitment or the Acquisition Facility, then, on demand by
Lender, Borrower further promises to pay to Lender, from time to time,
additional amounts which shall be sufficient to compensate Lender for the
portion of such increased costs allocable to this commitment or the Acquisition
Facility.  Written notice setting forth in reasonable detail such costs
incurred by Lender, submitted to Borrower from time to time, shall be
conclusive, absent manifest error, as to the amount thereof.

E.       CONDITIONS PRECEDENT

         In addition to the other requirements described herein, Lender's
obligation to sign loan documents amending or restating the Existing Credit
Agreement and make the Acquisition Facility available to Borrower is subject to
the following conditions:

                 (1)      a Qualified Offering shall have occurred and Borrower
         shall have received all net proceeds of such offering on or before
         November 30, 1995;
<PAGE>   7
Quest Medical, Inc.
September 25, 1995
Page 7


                 (2)      the outstanding principal balance of, and accrued
         unpaid interest on, and all fees related to, Facility B pursuant to
         the Existing Credit Agreement shall be paid in full and Facility B
         shall be terminated.

                 (3)      the documents and information previously delivered to
         Lender by Borrower shall be accurate, and there shall have been no
         material adverse changes in the matters covered by such documents and
         information prior to the execution of the loan documents; without
         limiting the foregoing, Borrower understands that Lender has relied
         upon documents and information furnished by Borrower to Lender
         regarding; the financial condition of Borrower and its subsidiaries
         (including financial statements dated as of December 31, 1994 and June
         30, 1995); financial projections for Borrower and its Subsidiaries;
         material contracts; and the existing and prospective management of
         Borrower and its Subsidiaries;

                 (4)      Lender shall be satisfied with regulatory matters
         affecting Borrower and its Consolidated Subsidiaries, including
         licenses and authorities to operate their businesses as previously and
         proposed to be conducted, the ability of Borrower to receive dividends
         from its direct and indirect subsidiaries, the absence of material
         regulatory violations, and, generally, the absence of material adverse
         regulatory events;

                 (5)      there shall have occurred no litigation or
         administrative proceedings, challenging the power or authority of
         Borrower or any of its Subsidiaries to operate their businesses as
         previously conducted and proposed to be conducted;

                 (6)      no Default or Event of Default under the Existing
         Credit Agreement shall have occurred; and

                 (7)      generally, no material adverse change shall have
         occurred int he business, financial condition, projections or
         prospects of Borrower and its Subsidiaries, taken as a whole.

F.       TERMINATION AND EXPIRATION

         1.      Expiration.  The proposal described above shall remain
effective until the close of business on September 29, 1995.  If Borrower
desires to accept this proposal, an authorized officer should execute a copy of
this letter and return it to the undersigned by such time, together with the
commitment fee described in Section A.6..

         2.      Termination.  If Borrower accepts this proposal, then the
commitment of Lender contained herein shall terminate upon the earliest to
occur of (a) January 15, 1996, (b) the occurrence of a material adverse change
in the financial condition of Borrower or its
<PAGE>   8
Quest Medical, Inc.
September 25, 1995
Page 8


subsidiaries, taken as a whole, and (c) the execution of definitive loan and
collateral documents pursuant to this Commitment.

G.       FINAL AGREEMENT

         THIS AGREEMENT REPRESENTS THE FINAL AGREEMENT BETWEEN THE PARTIES AND
MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT
ORAL AGREEMENTS BETWEEN THE PARTIES.  THERE ARE NO UNWRITTEN ORAL AGREEMENTS
BETWEEN THE PARTIES.

                                        Sincerely,
                                        
                                        NATIONSBANK OF TEXAS, N.A.
                                        
                                        
                                        By: /s/ BRIAN K. SCHNEIDER
                                           -----------------------------------
                                           Brian K. Schneider
                                           Vice President

AGREED TO AND ACCEPTED
THIS 25th DAY OF SEPTEMBER, 1995

QUEST MEDICAL, INC.


By: /s/ F. ROBERT MERRILL III                          
   -----------------------------------
   F. Robert Merrill III
   Vice President


