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Detect antibodies to nuclear antigens.
Males and females older than 80 years of age have a 50% incidence of low titer ANA. Various medications can induce a "lupoid" condition and elevated ANA titers. Usually, the titer decreases following removal of the drug.
The indirect immunofluorescent test has three elements to consider in the result:
1. Positive or negative fluorescence. A negative test is strong evidence against a diagnosis of SLE but not conclusive. See Anti-DNA (Single-stranded) Antibodies, Quantitative, IgG [161422].
2. The titer (dilution) to which fluorescence remains positive (provides a reflection of the concentration or avidity of the antibody). Many individuals, particularly the elderly, may have low titer ANA without significant disease substantiated after work-up.
3. The pattern of nuclear fluorescence (reflecting specificity for various diseases). Homogenous and/or nuclear rim (peripheral) pattern correlates with antibody to native DNA and deoxynucleoprotein and bears correlation with SLE, SLE activity, and lupus nephritis. Homogenous (diffuse) pattern suggests SLE or other connective tissue diseases. Speckled pattern correlates with antibody to nuclear antigens extractable by saline; it is found in many disease states, including SLE and scleroderma. When antibodies to DNA and deoxyribonucleoprotein are present (rim and homogenous pattern), there may be interference with the detection of speckled pattern. Nucleolar pattern is seen in sera of patients with progressive systemic sclerosis and Sjögren's syndrome. Centromere pattern is seen in CREST syndrome.
Five percent of the apparently "normal population" demonstrate serum ANA. Low titers of ANA reactivity may be seen in patients with rheumatoid arthritis (40% to 60% of patients), scleroderma (60% to 90%), discoid lupus, necrotizing vasculitis, Sjögren's syndrome (80%), chronic active hepatitis, pulmonary interstitial fibrosis, pneumoconiosis, tuberculosis, malignancy, age over 60 (18%), as well as in SLE, especially if the disease is inactive or under treatment. Titers ≥1:160 usually indicate the presence of active SLE, although occasionally other autoimmune disease may induce these high titers. There are now known groups of "ANA-negative" lupus patients. Such patients often have antibodies to SS-A/Ro antigen (usually when a frozen section substrate is used) and subacute cutaneous lupus. Ten percent of patients with SLE manifest biologic false-positive tests for syphilis; this may even be the initial manifestation. Some other tests used in differentiation of autoimmune states include antibody to double-stranded DNA, rheumatoid factor, antibody to extractable nuclear antigens, total hemolytic complement (C3, C4, etc). Although ANA tests are occasionally ordered on cerebrospinal fluid or synovial fluid, the current assays are not standardized for these fluids and such assays do not add to the diagnostic process.
