Twenty-eight years with antineutrophil cytoplasmic antibodies (ANCA): how to test for ANCA - evidence-based immunology?

Wegener's granulomatosis, microscopic polyangiitis, Churg-Strauss syndrome, and primary pauci-immune crescentic glomerulonephritis are associated with circulating antineutrophil cytoplasmic autoantibodies (ANCA) (collectively called ANCA-associated vasculitides, AAV). Two types of ANCA, one with a cytoplasmic fluorescence pattern (C-ANCA) and specificity for proteinase 3 (PR3-ANCA) and the other with a perinuclear pattern (P-ANCA) and specificity for myeloperoxidase (MPO-ANCA), account for this association and are highly specific markers for these vasculitides. AAV most often require therapy with cytotoxic and antiinflammatory agents, and hence a well-established diagnosis is mandatory to avoid unnecessary and risky treatment. The widespread use of ANCA screening in the past decade has resulted in the occurrence of greater numbers of false-positive results and has led to greater difficulty in test interpretation. Methods for ANCA detection have been standardized internationally in large multicentre studies and an international consensus statement on testing and reporting of ANCA has been pub lished (1999 and 2003). Despite these advances, problems with the extended use of ANCA testing in daily clinical practice remain. They may be summarized as follows: (1) the basic standards for ANCA testing are not uniformly met; (2) there is still controversy over the value of formalin fixation of neutrophils in differentiating P-ANCA from antinuclear antibodies (what is the place of this substrate in ANCA testing?); (3) the new generation of PR3-ANCA and MPO-ANCA ELISAs are more sensitive and specific than immunofluorescence testing (should ELISAs replace the immunofluorescence test?); and (4) should alternative methods for ANCA detection such as image analysis and/or multiplex immunoassays be used for screening? In this paper, we review these issues, identify areas of uncertainty, and provide practical guidelines where possible.