Because tests for rare diseases are likely to be falsely positive when ordered for the general population and the positive predictive value of a positive ANCA test is around 50%, ANCA testing should be reserved for patients in whom the pretest probability for GPA or another ANCA-associated vasculitis is at least moderately high (eg, patients with alveolar hemorrhage, glomerulonephritis, or multiple mononeuropathy plus other features of microscopic polyangiitis or GPA).