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Why Does My Rheumatologist Order An ANA Test?

 


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Antinuclear antibodies (ANA) are commonly seen in autoimmune diseases. While a positive ANA is not diagnostic of an autoimmune condition, it is seen often in diseases such as systemic lupus erythematosus (SLE), systemic sclerosis, Sjogren’s disease, polymyositis, and rheumatoid arthritis. Roughly 90 per cent of people with SLE will be ANA positive at some time during the course of their illness.

Who and where the ANA is performed is critically important. Many expert rheumatologists will have an office lab that is skilled in performing the ANA test properly. Often, commercial laboratories will have staff people who are not as experienced in ANA interpretation.

The ANA is a screening test that is very sensitive for the diagnosis of SLE. On the flip side, though, it is associated with many false positive test results, particularly when the ANA is at a low level. Usually, ANA levels of 1:80 or lower have less significance than higher levels do. However, the interpretation of the ANA must be made in combination with the patient’s history, physical examination, and other information in order to make a proper diagnosis.

ANAs also have patterns. These patterns sometimes point towards a diagnosis but are usually not specific. One pattern that seems to be relatively specific is the anti-centromere pattern which is seen in conditions such as systemic sclerosis or limited cutaneous sclerosis. The nucleolar pattern is also associated with Raynaud’s phenomenon and systemic sclerosis. Other patterns such as diffuse or speckled are not very specific. Rarely, a rim or peripheral pattern may be seen in patients with SLE.

If a patient has a positive ANA and other clinical signs, then more specific laboratory testing is required. Tests for these more specific antigens (proteins) are usually performed using what is called the ELISA technique. Antibodies to double-stranded DNA are fairly specific for SLE since 70 per cent of patients with SLE will have antibodies to double-stranded DNA at some point during their illness. High levels of antibodies to double-stranded DNA indicate more severe disease and also a higher likelihood of kidney disease. Measurements of antibodies to double stranded DNA change with disease activity so that this measurement should be repeated for monitoring purposes.

Anti-Sm antibody (anti-Smith) also is specific for SLE but is present in only about 30 per cent of patients with the disease. RNP antibodies are seen in patients who have a condition known as mixed connective tissue disease (MCTD).

Antibodies to SSA and SSB (also known as Ro and La) can be seen with primary Sjogren’s disease and SLE.

Other useful tests are antihistone antibodies which are seen with drug-induced lupus, anti-Scl-70 which is seen in systemic sclerosis, and anti-Jo-1 which is seen with dermatomyositis.

If these specific antibodies are performed as a quantitative measure, meaning a number representing the amount of antibody is given, then it is often useful to repeat these tests for monitoring purposes.

Where these more specific tests are performed is again important. If done in a laboratory where skilled technicians are performing the tests, then the reliability is much higher than if they are performed in a general commercial laboratory. Commercial laboratories handle a tremendous amount of volume as well and there have been instances when wrong results are given because of a mix up with specimens.

Nathan Wei, MD, FACP, FACR is a rheumatologist and Director of the Arthritis and Osteoporosis Center of Maryland (http://www.aocm.org ). He is a Clinical Assistant Professor of Medicine at the University of Maryland School of Medicine and consultant to the National Institutes of Health. For more info: Arthritis Treatment

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