/ɪˈlʌɪzə/

A sensitive immunological oassay that uses an enzyme linked to an antibody as a signal for the detection of a specific protein (an antigen or antibody). It is often used as a diagnostic test to determine exposure to a particular infectious agent, such as the AIDS virus.

There are indirect and direct ELISA:

An HIV ELISA for instance, sometimes called an HIV enzyme immunoassay (EIA) is the first and most basic test to determine if an individual is positive for a selected pathogen, such as HIV. The test is performed in a 8 cm x 12 cm plastic plate which contains an 8 x 12 matrix of 96 wells, each of which are about 1 cm high and 0.7 cm in diameter.

• Purified antigens or antibodies are pre-coated into a multi-well plate (ELISA plate). Wash coated wells if necessary.
• The sample (patient serum which may contain antigen or antibody that you are looking for) is loaded into the wells.
• After incubation unbound antigens or antibodies are washed.
• Solution with secondary antibody is loaded into wells. This antibody has an enzyme covalently attached. This enzyme modifies a molecule that generates colour (chromogen) once its modification takes place.

It is entirely possible that an individual not infected with HIV has antibodies which may give a positive result in the HIV ELISA. This is called a false positive. One reason for this is that people (especially women who have had multiple pregnancies) may possess antibodies directed against human leukocyte antigens (HLA) which are present on the host cells used to propagate HIV. As HIV buds from the surface of the host cell, it incorporates some of the host cell HLA into its envelope. False negatives can occur during the window between infection and an antibody response to the virus (seroconversion).

Above is ELISA data from three patients. Numbers are expressed as optical density at 450 nm. The cutoff value indicating a positive result is 0.500. Optical densities of 0.300 to 0.499 are indeterminate and need to be retested. Values below 0.300 are considered to be negative. In most cases, a patient will be retested if the serum gives a positive result. If the ELISA retests are positive, the patient will then be retested by western blotting analysis.

ELISA video