ELISA Tests to Diagnose Infectious Diseases
By Arlene
Schag
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.
