ELISA stands for enzyme-linked immuno assay. It is a commonly used laboratory test to detect antibodies in the blood.

Enzyme-linked immuno assay; EIA

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. This is called a venipuncture.

The sample is sent to a laboratory where the targeted antibody or antigen is linked to  specific enzyme. If the target substance is in the sample, the test solution turns a different color.

No special preparation is needed.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

This test is often used to see if you have been exposed to viruses or other substances that cause infection. It is often used to screen for current or past infections.

Normal values depend on the type of substance being identified. Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.

Abnormal values depend on the type of substance being identified. In some people, a positive result may be normal.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Ashihara Y, Kasahara Y, Nakamura RM. Immunoassay and immunochemistry. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 44.