What are Antinuclear Antibodies?

Antinuclear antibodies (ANA) attack proteins in the nucleus of regular cells. Antibodies are usually produced by the immune system to target foreign invaders such as bacteria for destruction, but sometimes the immune system makes mistakes and these antibodies attack normal parts of the body, such as cell nuclei. These antibodies are implicated in a range of autoimmune diseases but are also present in some people who are free from disease. A test for antinuclear antibodies is used in the diagnosis of certain diseases.

Antinuclear antibodies are a subset of the antibody group. Antibodies are proteins produced by white blood cells; once they recognize — or think they recognize — foreign material, they signal the body to begin the process of inflammation to try to kill the invader. Antinuclear antibodies mistakenly attack the proteins present in the part of the cell that contains genetic material called the nucleus.

These antibodies are implicated in a range of autoimmune diseases, which are illnesses caused by the immune system attacking its own body instead of foreign invaders. Antinuclear antibodies are found in people who have diseases such as lupus, rheumatoid arthritis, scleroderma, autoimmune hepatitis, Addison's disease and some diseases of the blood cells. Although a healthy person can have a certain level of these antibodies in his blood without suffering any ill effects, a high level can help doctors diagnose these illnesses.

An antinuclear antibody test involves taking blood samples from a patient. The blood sample is diluted with saline. In one such test, called the fluorescent antinuclear antibody test (FANA), the sample is mixed with fluorescent tags specific to the antibodies.

The analyst looks at the sample under a fluorescence microscope to detect the presence or absence of the tagged antibodies in the sample. The blood sample is diluted to reduce false positive results in the healthy people with low levels of ANA. In practice, a 1:40 dilution that gives a positive result does not indicate autoimmune disease, but a 1:160 dilution that has ANAs present does indicate disease.

A positive ANA test result is not used on its own to diagnose disease, because high levels of ANAs can be caused by reasons other than autoimmune disease. Certain drugs, such as phenytoin and procainamide, increase ANA levels. Cancer also can boost ANA readings, and even a viral infection can temporarily alter ANA levels. ANA levels also rise with age.