Is the ANA a good screening test for SLE or another autoimmune disease?
No. Simple mathematics indicates that if 5% of the US population is ANA-positive, then 16 million individuals will have a positive ANA. In contrast, the prevalence of SLE is only approximately 1/1000 (320,000 individuals with SLE in the United States). Thus, even if we presume 100% of SLE patients are ANA-positive, screening the entire population for ANA would result in many more normal individuals detected who are ANA-positive than SLE (i.e., 50 to 1). The clinical value of an ANA test is tremendously enhanced by ordering an ANA when there is a reasonable pre-test probability (i.e., clinical suspicion) of an autoimmune disease. Alternatively, a negative ANA (or 1:40 titer) makes it highly unlikely that the patient has SLE, mixed connective tissue disease (MCTD), Sjögren’s syndrome, or systemic sclerosis (high-sensitivity tests help rule out disease).