Immunoassays

These tests are most commonly used to screen samples. In the event that drugs or their metabolites are detected, then the sample is normally tested again using an even more sensitive test such as Gas Chromatography and Mass Spectrometry. Immunoassays work on the principle of antigen-antibody interaction. Antibodies are chosen which will bind selectively to drugs or their metabolites. The binding is then detected using either enzymes, radioisotopes or fluorescent compounds.

EMIT (Enzyme Multiplied Immunoassay Technique) is manufactured by Syva Laboratories. It uses an enzyme as the detection mechanism. It is the cheapest, simplest to perform and the most widely used of the immunoassays. Unfortunately, it is also the easiest to fail and more worryingly, the least accurate: giving a
4-34% false positive rate.

RIA (Radio Immunoassay) is manufactured by Roche Diagnostics. It is similar to EMIT but uses a radioactive isotope such as iodine instead of an enzyme. However, because it involves using radioactive substances, it is less popular than EMIT. This is a highly sensitive form of testing mainly used by the military.

FPI (Fluorescence Polarization Immunoassay) is manufactured by Abbott Laboratories. Fluorescent compounds mark the selective binding of antibodies to drugs and their metabolites. It is highly sensitive and highly specific.