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What Is a ELISA Test?

ELISA tests are highly sensitive and specific diagnostic tests for doctors to diagnose disease, infections and sensitivities. An ELISA test uses a patient's blood to measure specific components (called antigens) linked to a condition to provide quantitative levels. Various ELISA tests are available from laboratories such as HIV, Lyme disease, pregnancy, hepatitis, gastric ulcers or strep throat.

  1. Significance

    • An ELISA test is an enzyme-linked immunosorbent assay. It is also called an EIA test. ELISA tests are highly specific tests performed by a medical laboratory (assays) that provide measurements of concentrations of a substance (antigens) in blood (quantitative results). Levels are established that determine whether the antigen concentration differs from normal amounts, thereby indicating a positive or negative result. Since ELISA tests are very specific, these are effective in helping doctors diagnose infections such as HIV or Lyme disease, identify allergies or confirm pregnancy.

    Blood Sample

    • A health care professional or laboratory technician collects a blood sample from a vein into a vial using an aseptic technique (i.e., the area such as inside your elbow is cleaned before a sterile needle is inserted). The vial containing the blood sample is labeled and sent to a laboratory for analysis. The analysis involves seeing whether components in the blood react with a target substance (antibody) for a positive result, indicating that the patient has the antigen for the condition or disease. Sometimes a positive result is normal because a disease may affect normal body functions (so in this case, a positive result for the disease would be indicated by no reaction to the target substance).

    Assay

    • The ELISA test uses a plate containing multiple wells (microplate). Diluted samples are placed in the wells and a control sample is also dispensed for comparison. Tagged antibodies are added to the samples and excess solution is removed. The plate is processed in a machine that measures the amount of reaction that occurred through the level of tagged antibodies and antigens remaining. This analysis calculates the concentration of the substance found in the blood. The control sample provides a check that the test is performing correctly and it provides a known concentration for a positive or negative result.

    Accuracy

    • ELISA tests are very specific with antibodies to measure various substances in the blood. A vial of blood can be divided for multiple test samples because an indication of any number of substances may determine a positive result for a disease. The concentration measured by the analysis can provide health care professionals with the extent or severity of the condition.

    Considerations

    • A blood draw is necessary for an ELISA test. Advise your health care professional or lab technician collecting a blood sample if you have bleeding or clotting problems, or vein or blood pressure issues. Some patients have small veins that are hard to isolate for the blood draw and it may take longer to collect an appropriately sized sample.

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