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Elisa

Alternate Names

  • enzyme-linked immunosorbent assay

Definition

ELISA is test that is usually done on a sample of blood. This test may be used for various purposes. The test detects the presence of either antigens or antibodies in the blood. An antigen is a protein found in a substance in the body, such as a bacteria or piece of a protein. An antibody is a protein formed by the body in response to an antigen. Specific antigens "stick" to the specific antibodies that are created in response to them. Not all proteins in the body cause antibodies to be made. The proteins that do are possible candidates for an ELISA test.

Who is a candidate for the test?

A healthcare provider may want to order an ELISA test if he or she suspects a person has a certain infection, such as:
  • HIV, the virus that causes AIDS.
  • Lyme disease, a disease transmitted by ticks that can be caught while hiking or doing other outdoor activities. It can cause arthritis, neurologic problems, and heart problems.
  • H. pylori, a common cause of stomach ulcers.
  • chlamydia, a common cause of sexually transmitted disease.
  • rubella and toxoplasmosis, which can both cause serious birth defects in babies who are infected with this virus, while in the mother's womb.
  • mumps, a disease that is now uncommon because of the MMR vaccine that is given to all children in the US.
  • certain rare viruses, such as arboviruses, which can cause serious brain infections.
There are other infections that can be detected by this method as well.
ELISA can also be used to help in the diagnosis of blood clots, allergies, immune system diseases, and other conditions. The number of things being tested for with ELISA grows each year.

How is the test performed?

To perform this test, a blood sample is taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or tourniquet, is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them. A fine needle is gently inserted into a vein, and the rubber tube is removed. Blood flows from the vein through the needle, and is collected in a syringe or vial. After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.
Next, the blood sample is taken to the laboratory. In the laboratory, a test solution is combined with the blood sample. The test solution contains either an antibody or antigen coupled to an enzyme. For example, if the doctor wants to test for an antibody, the test solution will contain an antigen that will react with that antibody. An enzyme is a protein that speeds up a chemical reaction. If the substance being tested for is present, the test solution will react with the blood sample. This is because the antigen and antibody will combine. When the antigen and antibody combine, a chemical reaction occurs. In this reaction, the enzyme causes the test solution to change color, which is a positive result.

What is involved in preparation for the test?

A person should request specific instructions from his or her healthcare provider. Normally, no preparation is required.

What do the test results mean?

The results of the test are usually either positive or negative. In some cases, the test results are not clear. In this setting, a repeat test or an entirely different test may be needed to confirm or rule out the condition or infection.
If the test results are positive, the specific infection or condition being tested for may be present. For example, a positive result may indicate that a person has the HIV virus or has an allergy. The meaning of the results often depend on why the test was ordered. The healthcare provider generally discusses the meaning of the results.

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