Enzyme immunoassays (EIAs) use the catalytic properties of enzymes to detect and quantify immunologic reactions. Enzyme-linked immunosorbent assay (ELISA) is a heterogeneous EIA technique used in clinical analyses. In this type of assay, one of the reaction components is nonspecifically adsorbed or covalently bound to the surface of a solid phase, such as a microtiter well, a magnetic particle, or a plastic bead.