The diagnosis of systemic autoimmune rheumatic diseases (SARDs), such as systemic lupus erythematosus (SLE), is based on a constellation of clinical features and presence of supportive specific autoantibodies. The antinuclear antibody (ANA) test is the most common first‐line test for these groups of diseases. Although several methods can detect ANA, indirect immunofluorescence remains the most widely used assay and the method of choice.1 The most significant attribute of any screening test is its high negative predictive value; that is, a negative test rules out the possibility of these diseases. The ANA test fulfils this requirement, with a negative predictive value of about 99% for SLE and 95% for other diseases such as scleroderma, Sjögren syndrome and type 1 autoimmune hepatitis.2 However, the ANA test has a very poor positive predictive value of less than 10% in a community setting because healthy people might have a positive ANA result.3 The titre or concentration of antibody in the patient's serum is determined by serial doubling dilutions, starting from a dilution of 1:40 and then 1:80 up to 1:2560.3 Therefore, the use of higher screening dilutions can improve the positive predictive value of the ANA test. The potential clinical significance of a positive ANA test can be further determined by ordering for antibodies to extractable nuclear antigens (ENAs), extended ENA tests, and tests for double‐stranded DNA (dsDNA) antibodies.
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