AN AUDIT OF ANCA IN ROUTINE CLINICAL-PRACTICE

We have reviewed the medical records of 301/327 consecutive patients i n whom anti-neutrophil cytoplasmic antibodies (ANCA) were detected by the Regional Immunology Laboratory in Northern Ireland between January 1988 and October 1991 (45 months). We have collected data for each pa tient regarding age, sex, smoking habit, area of residence, and detail s of any other autoantibody activity. Clinical diagnosis was establish ed, with the number of organ systems involved and the evidence for tha t involvement (symptomatic, biochemical, radiological, and histologica l). Diagnoses were divided into four groups according to their recogni sed vasculitic features and these were related to the pattern of immun ofluorescence and maximum ANCA titre detected. The most frequent diagn osis was rheumatoid arthritis (18.2% of patients) and the connective t issue disorders as a whole accounted for 27.9% of patients. ANCA were also detected in a wide range of clinical conditions which are not ass ociated with vasculitis and these patients were an important source of 'false-positives'. The positive predictive value (PPV) of ANCA of all patterns and titres for vasculitic conditions was 27%, however, the d etection of a classical ANCA pattern at high titre (greater than or eq ual to 1:640) was associated with an increased PPV of 75%. The coexist ence of an antinuclear antibody (ANA) reduces the PPV of both classica l and perinuclear ANCA, although perinuclear ANCA with antimyeloperoxi dase specificity had an improved PPV. We conclude that ANCA testing sh ould not be used as the only screening investigation for vasculitis bu t should be in a rational investigative scheme. The interpretation of a positive ANCA result must take into account the presence of other au toantibodies and the full range of non-vasculitic conditions when the clinical situation is not typical of vasculitis.