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                                                                     Exhibit 4.3

THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES
ACT OF 1933, AS AMENDED (THE "ACT"), OR UNDER THE SECURITIES LAWS OF CERTAIN
STATES. THESE SECURITIES ARE SUBJECT TO RESTRICTIONS ON TRANSFERABILITY AND
RESALE AND MAY NOT BE TRANSFERRED OR RESOLD EXCEPT AS PERMITTED UNDER THE ACT
AND THE APPLICABLE STATE SECURITIES LAWS, PURSUANT TO REGISTRATION OR EXEMPTION
THEREFROM. HOLDERS SHOULD BE AWARE THAT THEY MAY BE REQUIRED TO BEAR THE
FINANCIAL RISKS OF THIS INVESTMENT FOR AN INDEFINITE PERIOD OF TIME. THE ISSUER
OF THESE SECURITIES MAY REQUIRE AN OPINION OF COUNSEL IN FORM AND SUBSTANCE
SATISFACTORY TO THE ISSUER TO THE EFFECT THAT ANY PROPOSED TRANSFER OR RESALE IS
IN COMPLIANCE WITH THE ACT AND ANY APPLICABLE STATE SECURITIES LAWS.


                       CONVERTIBLE BRIDGE PROMISSORY NOTE


$__________.00                                  Date of Issuance:  ____________



         Subject to the terms and conditions of this Bridge Note, for good and
valuable consideration received, CALYPTE BIOMEDICAL CORPORATION, a Delaware
corporation, (the "COMPANY"), promises to pay to TRILOBITE LAKES CORP., a
Delaware corporation (the "HOLDER"), whose address is Suite 14 Silverside
Carr Executive Center, Wilmington, Delaware 19089-1375., the principal amount of
___________________dollars ($__________.00), which principal amount the Company
acknowledges as having been received by the Company from the Holder on the date
hereof, plus simple interest accrued on unpaid principal from the date of this
Bridge Note until paid, at the rate of six and seventy-five one hundredths
percent (6.75%) per year.

         This promissory note is one of a series of Convertible Bridge
Promissory Notes (collectively, the "BRIDGE NOTES") totaling (as to this Bridge
Note and such other Bridge Notes together, regardless of when issued) up to a
maximum of one million dollars ($1,000,000.00) in principal amount, which other
Bridge Notes are identical to this Bridge Note except as to the principal
amount, interest rate, and issuance date thereof, that are issued pursuant to
that certain Bridge Loan Agreement dated as of March 2, 2000, to which the
Holder and the Company are parties (the "BRIDGE AGREEMENT").

         The following is a statement of the rights of the Holder and the terms
and conditions to which this Bridge Note is subject, to which the Holder, by
acceptance of this Bridge Note by such Holder's signature below, agrees:

         1.       PAYMENT.

                  (a) OBLIGATION. The outstanding principal under this Bridge
Note and the accrued interest thereon will be due and payable on May 30, 2000,
if prior thereto this Bridge Note is not converted, pursuant to Section 2
hereof, into Financing Automatic Conversion Shares (as defined in Section 2(a)
hereof). All payments of principal and/or interest under this Bridge Note will
be made at the address of the Holder set forth above or at such other address as
is provided by the Holder to the Company in writing.

                  (b) EQUAL RANKING OF BRIDGE NOTES. Each of the Bridge Notes,
including this Bridge Note, will rank equally among themselves as to payment. No
payment of any amount of principal and/or interest under this Bridge Note will
be paid by the Company to the Holder unless an amount, prorated among all of the
Bridge Notes in proportion to the principal amount of each Bridge Note, is
simultaneously paid to the Holder with respect to the other Bridge Notes.

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                  (c) PREPAYMENT. The Company may prepay this Bridge Note in
whole or in part at any time without penalty. Prepayments will be applied to
accrued but unpaid interest first and then to unpaid principal. Any prepayments
made by the Company on this Bridge Note or on any of the other Bridge Notes will
be made simultaneously on this Bridge Note and on all of the other Bridge Notes
in an amount prorated among all of the Bridge Notes in proportion to the
principal amount of each Bridge Note.

         2.       CONVERSION.

                  (a)      AUTOMATIC CONVERSION.

                           (i)      CERTAIN DEFINITIONS. For purposes of this
Bridge Note, (A) the "NEXT EQUITY FINANCING" means the sale by the Company of
shares of its Common Stock to a group of subscribing investors, under a Common
Stock Purchase Agreement dated as of March 2, 2000, having an aggregate purchase
price of all such shares so issued of up to twelve million dollars
($12,000,000.00), including in such aggregate amount the amount of this Bridge
Note and of all other Bridge Notes converted at such closing, and (B) the date
of the closing of the Next Equity Financing is hereinafter referred to as the
"FINANCING AUTOMATIC CONVERSION DATE".

                           (ii)     AUTOMATIC CONVERSION. If the Financing
Automatic Conversion Date occurs prior to the date upon which payment of this
Bridge Note is due, then on the Financing Automatic Conversion Date all
outstanding principal and accrued interest under this Bridge Note (and under all
other Bridge Notes) automatically will convert (the "FINANCING AUTOMATIC
CONVERSION"), on the Financing Automatic Conversion Date, into shares of Common
Stock of the Company (the "FINANCING AUTOMATIC CONVERSION SHARES"), at the same
price and on the same terms and conditions as the shares sold in the Next Equity
Financing.

                  (b)      MECHANICS AND EFFECT OF CONVERSION. The conversion of
this Bridge Note into the Financing Automatic Conversion Shares, pursuant to
Section 2(a) hereof, will be deemed to have been made immediately at the closing
of the Next Equity Financing on the Financing Automatic Conversion Date and the
Holder will be treated for all purposes as the record holder of such Financing
Automatic Conversion Shares on the Automatic Conversion Date. Upon such
conversion of this Bridge Note, the Company will be forever released from all of
its obligations and liabilities under this Bridge Note. No fractional shares
will be issued upon any such conversion of this Bridge Note. In lieu of any
fractional share to which the Holder would otherwise be entitled, the Company
will pay the cash value of that fractional share to the Holder, as provided
herein. The Company will give prompt written notice to the Holder and to the
holder of the other Bridge Note, of the proposed Financing Automatic Conversion
Date. The Company will furnish to the Holder, a reasonable period of time before
the proposed Financing Automatic Conversion Date, all agreements and materials
supplied to investors in the Next Equity Financing who are purchasing shares in
the Next Equity Financing in an aggregate purchase price equal to or greater
than the amount of principal and accrued interest to be converted hereunder (the
"FINANCING DOCUMENTS"). The Holder will surrender this Bridge Note on or before
the Financing Automatic Conversion Date, as set forth in the Company's notice
thereof, at the principal offices of the Company or its transfer agent together
with the executed signature page(s) to the stock purchase documents and
agreements applicable to the Next Equity Financing as requested by the Company.
The Company will, as soon as practicable thereafter, issue and deliver to the
Holder (i) a certificate or certificates for the number of Financing Automatic
Conversion Shares to which such Holder is entitled, bearing such legends as may
be required by applicable state and federal securities laws in the opinion of
legal counsel of the Company, and (ii) if applicable, a check payable to the
Holder for any cash amounts payable for any fractional share resulting from the
conversion of this Bridge Note.


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         3.       USE OF PROCEEDS. The full proceeds of this Bridge Note will be
used by the Company for operating capital (subject to the provisions of Section
4 of the Bridge Agreement).

         4.       GOVERNING LAW. This Bridge Note will be governed by and
construed in accordance with the laws of the State of California, excluding that
body of law relating to conflict of laws.

         5.       WAIVER. The Company hereby waives diligence, presentment,
demand, protest and notice of dishonor.

         6.       COLLECTION EXPENSES. If suit is brought for collection of this
Bridge Note, the Company will pay all reasonable expenses, including reasonable
fees and costs of attorneys, incurred by the Holder in connection therewith
whether or not such suit is commenced or is prosecuted to judgment.

                                           CALYPTE BIOMEDICAL CORPORATION



                                           By:
                                              ---------------------------------
                                                Name:
                                                     --------------------------
                                                Title:
                                                      -------------------------


AGREED AND ACCEPTED:

TRILOBITE LAKES CORP.

By:
   ---------------------------------
Name:
     -------------------------------
Title:
      ------------------------------
Date signed March __, 2000

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