Applications of ELISA

Because the ELISA can be performed to evaluate either the presence of antigen or the presence of antibody in a sample, it is a useful tool for determining serum antibody concentrations (such as with the HIV test or West Nile virus). It has also found applications in the food industry in detecting potential food allergens, such as milk, peanuts, walnuts, almonds, and eggs and as a serological blood test for coeliac disease.ELISA can also be used in toxicology as a rapid presumptive screen for certain classes of drugs.

The enzyme-linked immunosorbent assay plate
The ELISA was the first screening test widely used for HIV because of its high sensitivity. In an ELISA, a person's serum is diluted 400 times and applied to a plate to which HIV antigens are attached. If antibodies to HIV are present in the serum, they may bind to these HIV antigens. The plate is then washed to remove all other components of the serum. A specially prepared "secondary antibody" — an antibody that binds to other antibodies — is then applied to the plate, followed by another wash. This secondary antibody is chemically linked in advance to an enzyme.

Thus, the plate will contain enzyme in proportion to the amount of secondary antibody bound to the plate. A substrate for the enzyme is applied, and catalysis by the enzyme leads to a change in colour or fluorescence. ELISA results are reported as a number; the most controversial aspect of this test is determining the "cut-off" point between a positive and a negative result.

A cut-off point may be determined by comparing it with a known standard. If an ELISA test is used for drug screening at the workplace, a cut-off concentration, 50 ng/ml, for example, is established, and a sample containing the standard concentration of analyte will be prepared. Unknowns that generate a stronger signal than the known sample are "positive." Those that generate weaker signal are "negative".

Dr Dennis E Bidwell and Alister Voller created the ELISA test to detect various kinds of diseases, such as dengue, malaria, Chagas disease, and Johne's disease. ELISA tests also are used as in vitro diagnostics in medical laboratories. The other uses of ELISA include:

• detection of Mycobacterium antibodies in tuberculosis
• detection of rotavirus in feces
• detection of hepatitis B markers in serum
• detection of enterotoxin of E. coli in feces
• detection of HIV antibodies in blood samples