It is possible to determine low concentrations of compounds in biological fluids using the selectivity of immunological techniques, coupled to enzymatic colour-forming reactions. These EIA (Enzymze Immuno Assays) can be carried out using the competitive ELISA method (among others) described below.

The method works as follows:
the test substance is called the "antigen" (Ag), the antigen marked with enzyme is called AgE, and the antibody of this antigen is called Al.

A known quantity of AgE and an unknown (to be determined) quantity of unmarked antigen Ag are simultaneously brought into contact with a limited quantity of antibody, Al, which is bonded to a solid carrier.

AgE and Ag compete to bind to the antibody.

After equilibration the non-bonded antigen is rinsed off.
The enzyme substrate is added, forming a product that can be assayed using a colourimeter.

Results: much unknown Ag, little AgE binds, so very little coloured product; little Ag, much AgE binds, much coloured product.

If the colour reaction is calibrated, he concentration can be determined.