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Wednesday, April 16, 2008

Lab Tests for Lupus: The Antinuclear Antibody (ANA or FANA) Test

The immunofluorescent antinuclear antibody (ANA or FANA) test is a sensitive test for lupus, since it is present in 97 percent of those with the disease. When three or more typical clinical features are present, such as skin, joint, kidney, pleural, pericardial, hematological, or central nervous system findings as described above, a positive test confirms the diagnosis.

The ANA test is positive in almost all individuals with systemic lupus, and is the most sensitive diagnostic test currently available for confirming the diagnosis of systemic lupus when accompanied by typical clinical findings. A negative ANA test is strong evidence against lupus as the cause of a person's illness, although there are very infrequent instances where SLE is present without detectable anti-nuclear antibodies. ANA-negative lupus can be found in people who have anti-Ro (SSA) or antiphospholipid antibodies. However, a positive ANA test, by itself, is not proof of lupus since the test may also be positive in:

  • other connective tissue diseases such as scleroderma, Sjogren’s syndrome, rheumatoid arthritis, and thyroid disease, as well as liver disease and juvenile arthritis
  • individuals being treated with certain drugs, including procainamide, hydralazine, isoniazid, and chlorpromazine
  • viral illnesses such as infectious mononucleosis, and other chronic infectious diseases such as hepatitis, lepromatous leprosy, subacute bacterial endocarditis, and malaria
  • other autoimmune diseases, including thyroiditis and multiple sclerosis

The test can even be weakly positive in about 20 percent of healthy individuals. While a few of these healthy people may eventually develop lupus symptoms, the majority will never develop any signs of lupus or related conditions. The chances of a person having a positive ANA test increases as he or she ages.

Finally, as many as 30-40 percent of asymptomatic first degree relatives (siblings, parents, and children) of people with lupus may have a positive ANA test.

ANA Titers (number) and Patterns

ANA reports include a titer (pronounced TY-tur), and a pattern. The titer indicates how many times the lab technician had to dilute plasma from the blood to get a sample free of the antinuclear antibodies. For example, a titer of 1:640 shows a greater concentration of anti-nuclear antibodies than a titer of 1:320 or 1:160.

The apparent great difference between various titers can be misleading. Since each dilution involves doubling the amount of test fluid, it is not surprising that titers increase rather rapidly. In actuality, the difference between a 1:160 titer and a 1:320 titer is only a single dilution. This does not necessarily represent a major difference in disease activity. ANA titers go up and down during the course of the disease, and a high or low titer does not necessarily mean the disease is more or less active. Therefore, it is not always possible to determine the activity of the disease from the ANA titer.

A titer above 1:80 is usually considered positive. However, some laboratories may interpret different titer levels as positive, so one cannot compare titers from different laboratories.

The pattern of the ANA test can sometimes be helpful in determining which autoimmune disease is present and which treatment program is appropriate. The homogeneous (smooth) pattern is found in a variety of connective tissue diseases, as well as in people taking particular drugs such as certain anti-arrhythmics, anti-convulsants or anti-hypertensives. This pattern is also the one most commonly seen in healthy individuals who have positive ANA tests. The speckled pattern is found in SLE and other connective tissue diseases, while the peripheral (rim) pattern is found almost exclusively in SLE. The nucleolar (a pattern with a few large spots) pattern is found primarily in people who have scleroderma.

Because the ANA is positive in so many conditions, the results of the ANA test have to be interpreted in light of the person's medical history, as well as his or her clinical symptoms. Thus, a positive ANA alone is never enough to diagnose lupus. On the other hand, a negative ANA argues against lupus but does not rule out the disease completely.

A Positive ANA Does Not Equate to Having a Disease

The ANA should be looked at as a screening test. If it is positive in a person who is not feeling well and who has other symptoms or signs of lupus, the physician will probably want to conduct further tests for lupus. If the ANA is positive in a person who is feeling well and in whom there are no other signs of lupus, it can be ignored. If there is any doubt, a consultation with a rheumatologist should clarify the situation.

1 comments:

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