ELISA is the superior method for detecting antineutrophil cytoplasmic antibodies in the diagnosis of systemic necrotising vasculitis

Background-Antineutrophil cytoplasmic antibodies (ANCA) have been used as a diagnostic marker for systemic necrotising vasculitis, a disease classification which includes Wegener granulomatosis, microscopic and classic polyarteritis nodosa, and Churg Strauss disease. Objective-To compare the diagnostic value of the two methods for detecting these antibodies-immunofluorescence and enzyme linked immunosorbent assay (ELISA)-with respect to biopsy proven active systemic necrotising vasculitis in a clinically relevant population. Methods-A prospective study to ascertain the patient's diagnosis at the time of each of the 466 requests for ANCA received at one laboratory over a nine month period, and allocate each to one of five diagnostic groups: active and inactive biopsy proven systemic necrotising vasculitis, suspected systemic necrotising vasculitis, low probability systemic necrotising vasculitis, and not systemic necrotising vasculitis. Results-ELISA was superior to immunofluorescence in the diagnosis of systemic necrotising vasculitis because it was less likely to detect other diseases. This was reflected in its specificity of 97% and positive predictive value of 73%, compared with 90% and only 50% for immunofluorescence (p = 0.0006 and p = 0.013, respectively). ELISA had a negative predictive value of 98% which was not significantly different to immunofluorescence. ELISA was technically superior. Conclusions-ELISA is the superior method of ANCA detection in the diagnosis of systemic necrotising vasculitis and should be used in conjunction with a compatible clinical picture and histological evidence.