A positive ANA test result may suggest an autoimmune disease, but further specific testing is required to assist in making a final diagnosis. ANA test results can be positive in people without any known autoimmune disease. While this is not common, the frequency of a false positive ANA result increases as people get older.

Also, ANA may become positive before signs and symptoms of an autoimmune disease develop, so it may take time to tell the meaning of a positive ANA in a person who does not have symptoms. Most positive ANA results don't have significance, so physicians should reassure their patients but should also still be vigilant for development of signs and symptoms that might suggest an autoimmune disease.

About 95% of SLE patients have a positive ANA test result. If a patient also has symptoms of SLE, such as arthritis, a rash, and autoimmune thrombocytopenia, then he probably has SLE. In cases such as these, a positive ANA result can be useful to support SLE diagnosis. Two subset tests for specific types of autoantibodies, such as anti-dsDNA and anti-SM, may be ordered to help confirm that the condition is SLE.

A positive ANA can also mean that the patient has drug-induced lupus. This condition is associated with the development of autoantibodies to histones, which are water soluable proteins rich in the amino acids lysine and arginine. An anti-histone test may be ordered to support the diagnosis of drug-induced lupus.

Other conditions in which a positive ANA test result may be seen include:

Sjögren’s syndrome: Between 40% and 70% of patients with this condition have a positive ANA test result. While this finding supports the diagnosis, a negative result does not rule it out. The doctor may want to test for two subsets of ANA: Anti-SS-A (Ro) and Anti-SS-B (La). The frequency of autoantibodies to SSA in patients with Sjögren’s can be 90% or greater.
Scleroderma: About 60% to 90% of patients with scleroderma have a positive ANA finding. In patients who may have this condition, ANA subset tests can help distinguished two forms of the disease, limited versus diffuse. The diffuse form is more severe. Limited disease is most closely associated with the anticentromere pattern of ANA staining (and the anticentromere test), while the diffuse form is associated with autoantibodies to the anti–Scl-70.
A positive result on the ANA also may show up in patients with Raynaud’s disease, rheumatoid arthritis, dermatomyositis, mixed connective tissue disease, and other autoimmune conditions.
A doctor must rely on test results, clinical symptoms, and the patient’s history for diagnosis. Because symptoms may come and go, it may take months or years to show a pattern that might suggest SLE or any of the other autoimmune diseases.
Source(s):
http://www.labtestsonline.org/understanding/analytes/ana/test.html

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