The diagnostic value of anti-neutrophil cytoplasmic antibody testing in a routine clinical setting

Anti-neutrophil cytoplasmic antibody (ANCA) tests are a routine clinical as say in most UK hospitals. We examined the role of routine ANCA testing in a chieving 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 labor atory. After April 1999, enzyme-linked immunosorbent assays (ELISAs) were p erformed on all IIF-positive samples. Clinical diagnosis was determined for all patients with a positive IIF ANCA, and a sample of the ANCA-negative p atients. Some 2-18% of patients with suspected ANCA-associated systemic vas culitis (AASV) had positive IIF ANCA. The AASV diagnosis was confirmed in 0 -56% of these cases. Analysis by department suggested that 88-100% of patie nts 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 pro ven AASV, 41 % did not have ANCA on IIF. Combined ANCA testing by IIF/ELISA had a higher sensitivity and PPV but lower specificity than IF alone for A ASV. 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 w as 79% and the NPV was 63%. The ANCA test is being widely applied with very poor return. Guidelines for more effective usage are proposed.