Diseases

Health care personnel who perform the test are trained laboratory technicians who use special kits that measure the antigens' interactions with the antibodies in the kit. They will inform your doctor of the test results. Picture of micro-pipetting samples into a polystyrene plate for an ELISA test kit; Photo by James Gathany/CDC

In general, people do not need to prepare for an ELISA test. The test is done in a lab. If your blood is required, the only hurt is in blood collection. The risks associated with an ELISA test are rare and associated with blood withdrawal (infection, vessel damage, for example).

There are variations of the ELISA test (see below), but the most utilized type consists of an antibody attached to a solid surface (polystyrene plate). This antibody has affinity for (will latch on to) the substance of interest, such as a hormone, bacteria, or another antibody. For example, human chorionic gonadotropin hormone (HCG), the commonly measured protein which indicates pregnancy, can be detected by ELISA. A mixture of purified HCG linked to an enzyme and the test sample (blood or urine) are added to the test system. If no HCG is present in the test sample, then only the linked enzyme will bind to the solid surface. The more substance of interest that is present in the test sample, the less linked enzyme will bind to the solid surface. The more of the substance of interest is present it will cause a reaction and show up on the test plate in some way, such as a change in color of the solution (or like a pregnancy test "two pink lines" or a "+" mark).

Depending on what the test is being used for, you may get results as quickly as about 24 hours if the test is done locally. However, there are some tests that may take days to weeks.

ELISA tests are generally good and accurate tests. They are considered highly sensitive and specific (accurate) and compare favorably with other methods used for the detection of substances in the body. The ELISA testing method is more straightforward and easier to perform than older laboratory techniques, which often required radioactive materials.

There are four types or kinds of ELISA tests:

• Direct ELISA: attachment of an antigen to a polystyrene plate followed by an enzyme-labeled antibody that can react with the antigen and a substrate that can be measured
• Indirect ELISA: attachment of an antigen to a polystyrene plate followed by an unlabeled or primary antibody followed by an enzyme-labeled antibody that can react with both the primary antibody and substrate
• Sandwich ELISA: A capture antibody is attached to the polystyrene plate, then antigen is added that specifically attaches or captures the antigen. A second antibody, also specific for the antigen but not the same as the capture antibody is added and "sandwiches" the antigen. This second antibody is then followed by an enzyme-labeled antibody specific for the second antibody that can react with a substrate that can be measured
• Competitive ELISA: This test is like the sandwich ELISA but involves the addition of competing antibodies or proteins when the second antibody is added. This results in a decrease in the substrate signal that is generated. This test is considered to give good, highly specific results.

There are many hundreds of variations of ELISA tests. The results and their meaning depend on what is being tested. For example, an ELISA test for viral RNA can detect it (a positive test), not detect it (a negative test), or be indeterminate (borderline test). Rarely, it may result in a false negative or false positive result. If you have an ELISA test done, the best approach is to ask your physician what the results of the test mean to your individual situation.

An ELISA kit is a commercially available ELISA test that usually contains pre-coated polystyrene plates, detection antibodies, and usually all of the chemicals needed to perform an ELISA test. However, special kits can be purchased with substances designated by the customer.

An ELISA test uses components of the immune system (such as IgG or IgM antibodies) and chemicals for the detection of immune responses in the body (for example, to infectious microbes). The ELISA test involves an enzyme (a protein that catalyzes a biochemical reaction). It also involves an antibody or antigen (immunologic molecules). Examples of the uses of an ELISA test includes to diagnose infections such as HIV (human immunodeficiency virus) and some allergic diseases like food allergies. ELISA tests are also known as an immunosorbent assay.

ELISA is an abbreviation for "enzyme-linked immunosorbent assay."

ELISA tests are primarily used for the detection of proteins (as opposed to small molecules and ions such as glucose and potassium). The substances detected by ELISA tests can include hormones, viral antigens (dengue fever, for example), bacterial antigens (TB, for example), and antibodies that the body has made in response to infection (antibodies to hepatitis B, for example) or vaccination.