An integrated optical detection microsystem, which includes photodetectors and a light-to-frequency converter for readout, is designed and fabricated in a standard CMOS process without extra masks. This detection microsystem is designed for use in biological microsystems for fluids analysis. The application is in the low-cost concentration measurement of biomolecules in biological fluids, by the optical absorption in a part of the visible spectrum defined by the specific molecule. Signals proportional to the intensity of the light transmitted through the biological fluid are available at the output in the form of bit streams, which allows simple computer interfacing. The quantitative measurement of uric acid in urine is successfully demonstrated. The photodiode responsivity is 224 mA/W at λ= 495 nm (the wavelength at which the uric acid has its absorption maximum). The optical system sensitivity is 1 kHz/Wm-2 at λ =670 nm (using the TLS230 from Texas Instruments as reference).