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What is a ANA test?

Q: I want details about the Anti-nuclear and anti-smooth muscle antibody testing?

A:Anti-nuclear antibodies (ANA) are an unusual type of gamma-globulins found in patients with certain autoimmune diseases. ANAs are directed against certain components found in the nucleus of a cell in the body. They are found in patients whose immune system is predisposed to cause inflammation against their own body tissues (autoimmune diseases). An ANA test is needed if there is a reasonable suspicion of SLE from history, physical findings or the results of routine tests such as the CBC (leukopaenia, thrombocytopenia) as ANA may be found in a variety of conditions and in normal individuals. The random chance of a false positive in normal individuals is considerably greater than a true positive, unless the clinician selects only individuals with a reasonably high possibility of having the disease before testing. Therefore, careful selection of patients to be tested is of great importance as improper use of ANA testing can lead to misinformation, incorrect diagnoses and a waste of money. This is because, even with the newest tests, about 5% (1 out of 20) of normal individuals will give a weakly positive result and ANA should never be ordered as a routine screening test. A positive ANA result in a patient with no or minimal features of SLE can be misleading or worse because it may cause the clinician to order unnecessary tests. These antibodies can be detected by several techniques like enzyme-linked immunosorbent assay (ELISA), or immunofluorescence (IF). The ELISA test is a rapid semi-quantitative technique that determines the amount of the tested antibody in the serum. Serum anti-nuclear antibodies (ANA) bind to the corresponding antigens present in HEp-2 cells. The resulting antigen-antibody complexes are detected by means of a fluorescein labeled anti-human immunoglobulin, and visualized with the aid of a fluorescence microscope. The sensitivity of antinuclear antibodies determination is higher than 95% for SLE, although specificity is fairly low. The different patterns seen suggest a diagnosis: a) Homogeneous - Indicative of SLE; b) Peripheral - In patients with connective tissue diseases; c) Speckled - Highly related to SLE, mixed connective tissue disease, Sjögren's syndrome, polymyositis or scleroderma; d) Nucleolar - In approximately 50-70% of the patients with overlapping scleroderma and polymyositis/dermatomyositis syndromes. They are found in up to 33% of patients with systemic scleroderma, specially those with renal complications and e) Centromere - In patients with systemic sclerosis, especially in a cutaneous limited form of the disease (80%). Occasionally, in some other connective diseases. Anti-smooth muscle antibodies are seen in many patients with chronic active hepatitis, and some of those with forms of cirrhosis are positive for this test. Anti-smooth muscle antibodies can also be present in infectious mononucleosis. The test also helps distinguish autoimmune hepatitis from SLE. Anti-smooth muscle antibodies bind to muscle fibres in the arterials walls of the blood vessels in liver, kidney and stomach and these too can be detected either by IF or ELISA.


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