<PAGE>
                                                                     Exhibit 4.2

                         CALYPTE BIOMEDICAL CORPORATION

                              BRIDGE LOAN AGREEMENT


         This Agreement is made effective as of March 2, 2000 (the "EFFECTIVE
DATE") between Calypte Biomedical Corporation, a Delaware corporation (the
"COMPANY"), and Trilobite Lakes Corp., a Delaware corporation ("LENDER").

         1.       LENDER COMMITMENT. Lender hereby commits to loan to the
Company, on an unsecured basis, under the terms and conditions hereof, up to a
total in principal amount, for all amounts so loaned, of one million dollars
($1,000,000.00), with each such loan to be evidenced by a Convertible Bridge
Promissory Note (a "BRIDGE NOTE") of the Company to Lender in the form of
EXHIBIT A attached hereto and incorporated herein by reference. The Initial
Bridge Note, as defined in Section 2(a) hereof, as well as all Additional Bridge
Notes referred to in Section 2(c) hereof, will constitute the Bridge Notes.

         2.       INITIAL DRAWDOWN AND INITIAL BRIDGE NOTE; SUBSEQUENT DRAWDOWN
REQUESTS; ADDITIONAL BRIDGE NOTES; NO REBORROWING; EQUAL RANKING OF BRIDGE
NOTES; EXTENSION OF DUE DATE OF BRIDGE NOTES; INTEREST DURING EXTENSION.

                  (a)      INITIAL DRAWDOWN AND INITIAL BRIDGE NOTE. As soon
as practicable following the execution of this agreement, Lender will loan to
the Company five hundred thousand dollars ($500,000.00) in principal amount,
as the initial drawdown by the Company from Lender pursuant to this
Agreement, receipt of which funds the Company acknowledges, in exchange for
the issuance by the Company to Lender of a Bridge Note (the "INITIAL BRIDGE
NOTE") in the form attached hereto as Exhibit A, receipt of which executed
Initial Bridge Note is acknowledged by Lender.

                  (b)      SUBSEQUENT DRAWDOWN REQUESTS. Amounts loaned to the
Company by Lender pursuant to this Agreement, beyond the initial loan by Lender
to the Company pursuant to this Agreement evidenced by the Initial Bridge Note,
will be made only in response to a written request of the Company to Lender
therefor (each such request referred to herein as a "DRAWDOWN REQUEST"). Each
Drawdown Request will (i) be given at least five (5) business days (defined as a
day when U.S. national banks are open for business) prior to the date upon which
such funds are requested, and (ii) be for a minimum of one hundred thousand
dollars ($100,000.00) or such lesser amount as is available after subtracting,
from the one million dollar ($1,000,000.00) maximum aggregate principal amount
limit set forth in Section 1 hereof, the aggregate principal amount of all
then-outstanding Bridge Notes.

                  (c)      ADDITIONAL BRIDGE NOTES. Each such additional loan
will be made under additional Convertible Promissory Notes of the Company
(the "ADDITIONAL BRIDGE NOTES") delivered by the Company, at the date of such
subsequent borrowing, to the Lender, which will be identical in form to the
Initial Bridge Note except as to the date(s) and principal amount(s) thereof,
and, as applicable, the interest rate thereof. As will be provided in the
Additional Bridge Notes, (i) the Bridge Notes will rank equally among
themselves as to all rights, (ii) all Bridge Notes will have the same due
date, which will be May 30, 2000, and (iii) the interest rate for such
Additional Bridge Notes will be 6.75%. Each such Additional Bridge Note will
be deemed subject to this Agreement upon its issuance.

<PAGE>


                  (d)      MAXIMUM TOTAL PRINCIPAL AMOUNT; NO REBORROWING. The
maximum total principal amount under all outstanding Bridge Notes, together,
will not exceed one million dollars ($1,000,000.00). No amount repaid by the
Company under any of the Bridge Notes may be reborrowed by the Company from the
Lender.

                  (e)      EQUAL RANKING OF BRIDGE NOTES. Without the express
prior written consent of Lender, (i) each of the Bridge Notes will rank equally
(PARI PASSU) among themselves as to payment, and (ii) no payment of any amount
of principal and/or interest under any Bridge Note will be paid by the Company
to Lender unless an amount prorated among the Bridge Notes in proportion to the
principal amounts of each Bridge Note is simultaneously paid by the Company to
Lender.

         3.       BRIDGE LOAN WARRANT; REGISTRATION ON FORM S-3; SEC REPORTING
AND RELATED MATTERS.

                  (a)      BRIDGE LOAN WARRANT. On the Effective Date, the
Company has issued to Lender a Warrant (the "BRIDGE LOAN WARRANT") in the form
of EXHIBIT B attached hereto and incorporated herein by reference.

                  (b)      REGISTRATION OF BRIDGE LOAN WARRANT SHARES ON FORM
S-3; TRANSFER OF BRIDGE LOAN WARRANT SHARES AFTER REGISTRATION; SEC REPORTING
AND RELATED MATTERS.

                           (i)      REGISTRATION OF BRIDGE LOAN WARRANT SHARES
ON FORM S-3. The Company will do the following:

                                    (A)      As soon as practicable following
the Effective Date, the Company will include the shares for which the Bridge
Loan Warrant is exerciseable (the "BRIDGE LOAN WARRANT Shares") in a
registration statement (the "REGISTRATION STATEMENT") which the Company will
file with the Securities and Exchange Commission (the "SEC") in connection with
a Common Stock Purchase Agreement dated as of the Effective Date (the "COMMON
STOCK PURCHASE AGREEMENT"), which agreement provides for the sale of shares of
the Company's Common Stock (the "FINANCING SHARES") to certain investors. The
Company will use its best efforts to cause such Registration Statement to be
declared effective as soon thereafter as reasonably possible.

                                    (B)      The Company will prepare and file
with the SEC (1) such amendments and supplements to the Registration Statement
and the prospectuses used in connection therewith, (2) such SEC reports and
filings required to be filed by the Company, and (3) such other filings required
by the SEC, in each case as may be necessary to keep the Registration Statement
continuously effective and not misleading until the earliest of (a) the second
anniversary date of the date of the closing of the Common Stock Purchase
Agreement, (b) such date as all of the Financing Shares have been resold or (c)
such time as all of the Financing Shares held by the holders thereof under the
Common Stock Purchase Agreement can be sold within a given three-month period
pursuant to Rule 144 under the Securities Act.


                                      -2-
<PAGE>

Notwithstanding the foregoing, following the effectiveness of the Registration
Statement, the Company may, at any time, suspend the effectiveness of the
Registration Statement for up to no longer than 30 days, as appropriate (a
"SUSPENSION PERIOD"), by giving notice to Lender, if the Company has determined
that the Company may be required to disclose any material corporate development.
The Company will use its best efforts to minimize the length of any Suspension
Period. Notwithstanding the foregoing, the Company may not suspend the
effectiveness of the Registration Statements more than twice in any twelve (12)
month period. Lender agrees that, upon receipt of any notice from the Company of
a Suspension Period, Lender will not sell any of the Bridge Loan Warrant Shares
pursuant to the Registration Statement until (1) Lender is advised in writing by
the Company that the use of the applicable prospectus may be resumed, (2) Lender
has received copies of any additional or supplemental or amended prospectus, if
applicable, and (3) Lender has received copies of any additional or supplemental
filings which are incorporated or deemed to be incorporated by reference in such
prospectus.

                                    (C)      In order to facilitate the public
sale or other disposition by Lender of all or any of the Bridge Loan Warrant
Shares, the Company will furnish to Lender, with respect to the Bridge Loan
Warrant Shares which have been registered under the Registration Statements,
such number of copies of prospectuses, prospectus supplements and preliminary
prospectuses as Lender reasonably requests in conformity with the requirements
of the Securities Act of 1933, as amended (the "SECURITIES ACT").

                                    (D)      The Company will file any documents
required of the Company for normal blue sky clearance in states specified in
writing by Lender; provided, however, that the Company will not be required to
qualify to do business in any jurisdiction in which it is not now so qualified.

                                    (E)      Other than fees and expenses, if
any, of counsel or other advisers to Lender, which fees and expenses will be
borne by Lender, the Company will bear all expenses, (exclusive of any brokerage
fees, underwriting discounts and commissions) in connection with the procedures
in this Section 3(c).

                                    (F)      With a view to making available to
Lender the benefits of Rule 144 promulgated under the Securities Act ("RULE
144") and any other rule or regulation of the SEC that may at any time permit a
Purchaser to sell the Bridge Loan Warrant Shares to the public without
registration or pursuant to registration, the Company covenants and agrees to:
(1) make and keep public information available, as those terms are understood
and defined in Rule 144, until the earlier of (a) the second anniversary of the
Closing Date or (b) such date as all of the Bridge Loan Warrant Shares have been
resold; (2) file with the SEC in a timely manner all reports and other documents
required of the Company under the Securities Exchange Act of 1934 (the "EXCHANGE
ACT"); and (3) furnish to any Purchaser upon request, as long as the Purchaser
owns any Bridge Loan Warrant Shares, (a) a written statement by the Company that
it has complied with the reporting requirements of the Exchange Act, (b) a copy
of the most recent annual or quarterly report of the Company, and (c) such other
information as may be reasonably requested in order to avail any Purchaser of
any rule or regulation of the SEC that permits the selling of any such Bridge
Loan Warrant Shares without registration under the Securities Act.

                           (ii)     TRANSFER OF BRIDGE LOAN WARRANT SHARES AFTER
REGISTRATION. Lender agrees that Lender will not effect any disposition of the
Bridge Loan


                                      -3-
<PAGE>

Warrant Shares that would constitute a sale within the meaning of the Securities
Act, except:

                                    (A)      pursuant to the applicable
Registration Statement, in which case Lender will submit the certificates
evidencing the Bridge Loan Warrant Shares to the Company's transfer agent,
accompanied by a separate certificate (1) in customary form as will be required
for such transfer of any of the Financing Shares under the Common Stock Purchase
Agreement, (2) executed by Lender or by an officer of, or other authorized
person designated by, Lender, and (3) to the effect that (a) the Bridge Loan
Warrant Shares have been sold in accordance with the applicable Registration
Statement and (b) the requirement of delivering a current prospectus has been
satisfied; or

                                    (B)      in a transaction exempt from
registration under the Securities Act, in which case Lender will, prior to
effecting such disposition, submit to the Company an opinion of counsel in form
and substance reasonably satisfactory to the Company to the effect that the
proposed transaction is in compliance with the Securities Act.

                           (iii)    SEC REPORTING AND RELATED MATTERS. The
Company will, at all times prior the declaration of effectiveness of the Form
S-3 covering the public resale of the Bridge Loan Warrant Shares:

                                    (A)      use its best efforts to make and
keep public information available, as those terms are understood and defined in
Rule 144, as promulgated under the Securities Act;

                                    (B)      use its best efforts to file with
the SEC in a timely manner all reports and other documents required of the
Company under the Securities Act and the Exchange Act;

                                    (C)      furnish to Lender, so long as
Lender owns any Bridge Loan Warrant Shares, forthwith upon request (1) a written
statement by the Company as to the Company's compliance with the reporting
requirements of Rule 144, the Securities Act and the Exchange Act (during such
time after the Company is subject to such reporting requirements), or as to its
qualification as a registrant whose securities may be resold pursuant to Form
S-3 (at any time it so qualifies), (2) a copy of the most recent annual or
quarterly report of the Company and such other reports and documents so filed by
the Company, and (3) such other information as may be reasonably requested in
availing Lender of any rule or regulation of the SEC which permits the selling
of any such securities without registration or pursuant to such form; and

                                    (D)      at all times during which the
Company is neither subject to the reporting requirements of Section 13 or 15(d)
of the Exchange Act, provide in written form, upon the written request of
Lender, or a prospective purchaser or securities of the Company from Lender, all
information required by Rule 144A(d)(4)(i) of the General Regulations
promulgated by the SEC under the Securities Act ("144A INFORMATION"); the
Company further agrees, upon written request, to cooperate with and assist
Lender or any member of the National Association of Securities Dealers, Inc.
system for Private Offerings Resales and Trading through Automated Linkages
("PORTAL") in applying to designate and thereafter maintaining the eligibility
of the Company's securities for trading through


                                      -4-
<PAGE>

PORTAL. The Company's obligations under this Section 3(c)(iii) will at all times
be contingent upon Lender's obtaining from a prospective purchaser an agreement
to take all reasonable precautions to safeguard the 144A Information from
disclosure to anyone other than employees of the prospective purchaser who
require access to the 144A Information for the sole purpose of evaluating its
purchase of the Company's securities.

         4.       USE OF PROCEEDS. All proceeds to the Company from the Bridge
Notes will be used by the Company only for operating capital, including regular
payments by the Company of debt of the Company to institutional or other
financial lenders to the Company incurred (a) prior to the Effective Date or (b)
after the Effective Date, in each case in the ordinary course of the Company's
business.

         5.       MISCELLANEOUS.

                  (a)      COUNTING OF TIME. Whenever days are to be counted
under this Agreement, the first day will not be counted and the last day will be
counted.

                  (b)      NOTICES. All notices, requests, consents and other
communications hereunder to any party will be deemed to be sufficient if
contained in a written instrument delivered in person, including delivery by
recognized express courier, fees prepaid, or sent by facsimile transmission or
duly sent by first class registered or certified mail, return receipt requested,
postage prepaid, in each case addressed as set forth below such party's
signature below, or to such other address as may hereinafter be designated in
writing by the recipient to the sender pursuant to this Section 4(b). All such
notices, requests, consents and other communications will be deemed to have been
received in the case of personal delivery, including delivery by express
courier, on the date of such delivery; in the case of facsimile transmission, on
the date of transmission; and in the case of mailing, on the third day after
deposit in the U.S. mail, proper postage prepaid.

                  (c)      TERMINATION. When the Company has repaid to Lender
all amounts owed to Lender under all Bridge Notes, whether by payment in cash,
or conversion to equity securities of the Company, or any combination thereof,
this Agreement will terminate automatically without the need for any other or
further signature of any party or any other action.

                  (d)      AMENDMENTS AND WAIVERS. Neither this Agreement or any
term hereof may be amended, modified or waived, except by written instrument
signed by the Company and by Lender. No course of dealing between the parties
will operate as a waiver of either party's rights under this Agreement. A waiver
on any one occasion will not be construed as a bar to or waiver of any right or
remedy on any future occasion. The Company acknowledges that the giving by
Lender of any notice or information to the Company, or the securing of any
consent by the Company, not required by the express terms hereof to be given or
secured, will not by implication constitute an amendment to or waiver or
modification of any provision hereof, or impose upon Lender any duty to give any
such notice or information or to secure any such consent on any future occasion.

                  (e)      ATTORNEYS' FEES. If either party hereto commences or
maintains any action at law or in equity (including counterclaims or
cross-complaints) against the other


                                      -5-
<PAGE>

party by reason of the breach or claimed breach of any term or provision of this
Agreement by such other party, then the prevailing party in such action will be
entitled to recover its reasonable attorney's fees and court costs incurred
therein from the other party to such action.

                  (f)      SUCCESSORS AND ASSIGNS. The provisions of this
Agreement will inure to the benefit of, and be binding on, each party's
respective successors and assigns. This Agreement is not assignable in whole or
in part by either party hereto without the prior written consent of both
parties, which will not be unreasonably withheld or delayed.

                  (g)      SEVERABILITY; ENTIRE AGREEMENT. The invalidity or
unenforceability of any term or provision of this Agreement will not affect the
validity or enforceability of any other term or provision hereof. The headings
in this Agreement are for convenience of reference only and will not alter or
otherwise affect the meaning of this Agreement. This Agreement and the Bridge
Notes, and the Exhibits hereto and thereto, together constitute the entire
agreement of the parties regarding the specific subject matter hereof and
supersede any and all prior understandings or agreements between the parties
with respect to such specific subject matter.

                  (h)      GOVERNING LAW. This Agreement will be governed by and
construed in accordance with the laws of the State of California, excluding that
body of law pertaining to conflict of laws or choice of law.

                           (i)      EXECUTION IN COUNTERPARTS. This Agreement
may be executed in counterparts, which together will constitute one instrument.

CALYPTE BIOMEDICAL CORPORATION                TRILOBITE LAKES CORP.

By:                                           By:
   ----------------------------------            -------------------------------
Name:                                         Name:
     --------------------------------              -----------------------------
Title:                                        Title:
      -------------------------------               ----------------------------
Date signed:  March __, 2000                  Date signed:  March __, 2000
Address:  Attention: President                Address:  Attention:
          1265 Harbor Bay Parkway                                 --------------
          Alameda, CA 94502
                                              ----------------------------------

                                              ----------------------------------
Fax:      (510) 814-8403                      Fax:        (___)-___-____


                                      -6-
<PAGE>

                                    EXHIBIT A

                               FORM OF BRIDGE NOTE


<PAGE>

                                    EXHIBIT B

                           FORM OF BRIDGE LOAN WARRANT