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 classific ation which includes Wegener granulomatosis, microscopic and classic polyar teritis 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 tim e of each of the 466 requests for ANCA received at one laboratory over a ni ne month period, and allocate each to one of five diagnostic groups: active and inactive biopsy proven systemic necrotising vasculitis, suspected syst emic necrotising vasculitis, low probability systemic necrotising vasculiti s, and not systemic necrotising vasculitis. Results-ELISA was superior to immunofluorescence in the diagnosis of system ic necrotising vasculitis because it was less likely to detect other diseas es. This was reflected in its specificity of 97% and positive predictive va lue of 73%, compared with 90% and only 50% for immunofluorescence (p = 0.00 06 and p = 0.013, respectively). ELISA had a negative predictive value of 9 8% which was not significantly different to immunofluorescence. ELISA was t echnically 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.