Of the many methods available for antigen and antibody detection, LA, ELISA, RIA, CIE, and COA are the most widely used.Enzyme immunoassays (EIAs) can be either homogeneous (EMIT) or heterogeneous (ELISA). EMIT has been used primarily for assays of low- molecular-weight drugs. Its primary use in microbiology has been for assays of aminoglycoside antibiotics. EIAs vary as to the solid support used. A variety of supports can be used, such as polystyrene microdilution plates, paddles, plastic beads, and tubes. The number of layers in the antibody-antigen sandwich varies; usually as additional layers are added, detection sensitivity is increased. The two most common enzymes are horseradish peroxidase (HRP) and alkaline phosphatase (AP). b-Galactosidase has also been employed. O-phenylenediamine is the most common substrate for HRP and b-nitrophenyl phosphate for AP. Because EIAs are usually read in the visible color range, the tests can be read qualitatively by eye or quantitatively by machine.