An ELISA test is used to help diagnose many different kinds of diseases in humans, animals and even
plants. The test (or assay) is an acronym that stands for Enzyme-Linked ImmunoSorbent Assay. Sometimes
it's just called an Enzyme-Immuno Assay, or EIA. The general public may hear the ELISA test called an
Antigen Detection Test too. It is similar to a how a pregnancy test works, when a chemical embedded on a
dipstick helps you see if you or your partner is pregnant.

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

The most common ELISA test comes in on a plate with a 8 X 12 matrix of 96 microwells. For example, an
ELISA test to determine the presence of HIV infection may be coated with anti-HIV antibodies. Patient blood
(serum) which contains antibodies is applied to the microwells. If the patient is HIV+, then this serum will
contain antibodies to HIV, and those antibodies will bind to the HIV antigens on the plate. An anti-human
immunoglobin attached to an enzyme is applied and it it binds to the first antibodies. Chromogen or
substrate is added that changes color when cleaved by the enzyme attached to the second antibody. Thus,
the microwells change color and visually represent the presence of infection.

The ELISA test method is not without limitations. A nonreactive test result does not exclude the possibility of
exposure to a pathogen. Levels of antibodies may be below the detectable limit of the assay or
undetectable during an early stage following exposure.
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