Antinuclear Antibody Test
In autoimmune diseases, the body makes antibodies that work against its own cells or tissues. Rheumatic diseases (diseases that affect connective tissue, including the joints, bone, and muscle) are also associated with these antibodies. Autoantibodies are proteins built by the body, but instead of guarding against foreign material (including bacteria, viruses, and fungi) as normal antibodies do, they attack the body's own cells.
Autoimmune and rheumatic diseases can be difficult to diagnose. People with the same disease can have very different symptoms. A helpful strategy in the diagnosis of these diseases is to find and identify an autoantibody in the person's blood.
The antinuclear antibody test looks for a group of autoantibodies that attack substances found in the center (nucleus) of all cells. It is useful as a screen for many autoantibodies associated with diseases that affect the entire body (systemic diseases).
This test is particularly useful when diagnosing a person with symptoms of SLE, an illness that affects many body organs and tissues. If the test is negative, it is unlikely that the person has SLE; if the test is positive, more tests are done to confirm whether the person has SLE or another related disease. Other diseases, such as scleroderma, Sjögren's syndrome, Raynaud's disease, rheumatoid arthritis, and autoimmune hepatitis, often have a positive test for antinuclear antibodies.
Five to 10 mL of blood is needed for this test. The antinuclear antibody test is done by adding a person's serum to commercial cells mounted on a microscope slide. If antinuclear antibodies are in the serum, they bind to the nuclei of cells on the slide. Next, a second antibody is added to the mixture. This antibody is "tagged" with a fluorescent dye so that it can be seen. The second antibody attaches to any antibodies and cells bound together and, because of the fluorescent "tag," the areas with antinuclear antibodies seem to glow, or fluoresce, when the slide is viewed under an ultraviolet microscope.
If fluorescent cells are seen, the test is positive. When positive, the serum is diluted, or titered, and the test done again. These steps are repeated until the serum is so dilute it no longer gives a positive result. The last dilution that shows fluorescence is the titer reported.
The pattern of fluorescence within the cells gives the physician clues as to what the disease might be. The test result includes the titer and the pattern.
This test is also called the fluorescent antinuclear antibody test or FANA. Results are available within one to three days.
Discomfort or bruising may occur at the puncture site or the person may feel dizzy or faint. Pressure to the puncture site until the bleeding stops reduces bruising. Warm packs relieve discomfort.
Normal results will be negative, showing no antinuclear antibodies.
A positive test in a person with symptoms of an autoimmune or rheumatic disease helps the physician make a diagnosis. More than 95% of people with SLE have a positive ANA test. Scleroderma has a 60-71% positive rate; Sjögren's disease, 50-60%, and rheumatoid arthritis, 25-30%.
Several factors must be considered when interpreting a positive test. Diseases other than autoimmune diseases can cause autoantibodies. Some healthy people have a positive test. More testing is done after a positive test to identify individual autoantibodies associated with the various diseases.
Clinical Diagnosis and Management by Laboratory Methods. 19th ed. Ed. John B. Henry. Philadelphia: W. B. Saunders Co., 1996.
Lehman, Craig A. Saunders Manual of Clinical Laboratory Science. Philadelphia: W. B. Saunders Co., 1998.
A Manual of Laboratory and Diagnostic Tests. 5th ed. Ed. Francis Fishback. Philadelphia: Lippincott, 1996.
Mayo Medical Laboratories. Interpretive Handbook. Rochester, MN: Mayo Medical Laboratories, 1997.
Pagana, Kathleen Deska. Mosby's Manual of Diagnostic and Laboratory Tests. St. Louis: Mosby, Inc., 1998.
Slater, Cindi S., Roger B. Davis, and Robert H. Shmerling. "Antinuclear Antibody Testing. A Study of Clinical Utility." Archives of Internal Medicine (July 1996): 1421-1425.
Ward, Michael M. "Laboratory Testing for Systemic Rheumatic Diseases." Postgraduate Medicine (Feb. 1998): 93-100.
Nancy J. Nordenson
Autoantibody—An antibody that attacks the body's own cells or tissues.
Antinuclear antibodies—Autoantibodies that attack substances found in the center, or nucleus, of all cells.
Autoimmune disease—Disease in which the body makes antibodies against its own cells or tissues.
Titer—A dilution of a substance with an exact known amount of fluid. For example, one part of serum diluted with four parts of saline is a titer of 1:4.