Fluorescence Polarization Immunoassay Drug Test (aka “FPI” Drug Test Method)

This drug screen, distributed widely by Abbot labs, utilizes fluorescence polarization to determine the presence of drugs.

Fluorescent molecules excited by plane polarized light emit light in the same polarized plane if the molecule remains stationary through the excited state. If the molecule tumbles out of the plane during the excited state light is emitted in a different plane from the initial excitation state. By using vertically polarized light the emitted light intensity can be viewed in two planes, horizontal and vertical. The shift between the two planes indicates the mobility of the molecule. The larger the molecule, the slower it moves and the light remains polarized. Small, fast moving molecules result in light being emitted in a depolarized state relative to the excitation stage.

A labeled drug is added to the urine sample. The lower the amount of the drug in the specimen, the more of the added labeled drug is allowed to bind. The antibody attaches to the molecule, which in turn moves more slowly and the fluorescence polarization is higher.

A curve is constructed in which polarization levels are compared to the levels in known drug concentrations and the concentration of drugs in the sample specimens are easily determined.