Anti-neutrophil cytoplasmic antibody (ANCA) tests are a routine clinical assay in most UK hospitals. We examined the role of routine ANCA testing in achieving a diagnosis of systemic vasculitis in a routine clinical setting. From April 1996 to March 2000, 2734 samples from five hospital departments were tested for ANCA by indirect immunofluorescence (IIF) at a single laboratory. After April 1999, enzyme-linked immunosorbent assays (ELISAs) were performed on all IIF-positive samples. Clinical diagnosis was determined for all patients with a positive IIF ANCA, and a sample of the ANCA-negative patients. Some 2-18 of patients with suspected ANCA-associated systemic vasculitis (AASV) had positive IIF ANCA. The AASV diagnosis was confirmed in 0-56 of these cases. Analysis by department suggested that 88-100 of patients with a positive IIF ANCA did not have AASV, except in the Rheumatology department. The positive predictive value (PPV) of IIF ANCA for AASV was 59 and the negative predictive value (NPV) was 84. Of the patients with proven AASV, 41 did not have ANCA on IIF. Combined ANCA testing by IIF/ELISA had a higher sensitivity and PPV but lower specificity than IIF alone for AASV. For the combined IIF/ELISA test, only the Rheumatology department had a sensitivity or PPV >0 for AASV. The PPV of ANCA by IIF/ELISA for AASV was 79 and the NPV was 63. The ANCA test is being widely applied with very poor return. Guidelines for more effective usage are proposed.
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