MPO-C-ANCA Renal-Limited Vasculitis: A Puzzling Phenomenon

Antineutrophil cytoplasmic antibodies (ANCAs) are autoantibodies directed against antigens in the cytoplasm of neutrophils and can be used diagnostically to identify different types of systemic vasculitides such as microscopic polyangiitis (MPA), granulomatosis with polyangiitis (GPA), eosinophilic granulomatosis with polyangiitis (EGPA), and renal-limited vasculitis (RLV). Two methods are used concomitantly to detect ANCAs: indirect immunofluorescence (IIF) and enzyme-linked immunosorbent assay (ELISA). IIF yields either a cytoplasmic (C-ANCA) or peri-nuclear (P-ANCA) pattern with a corresponding antigen identified on ELISA; either proteinase-3 (PR-3) or myeloperoxidase (MPO), respectively. Here, we present a 76 year-old female with a presenting complaint of nausea and emesis, ultimately found to have renal-limited MPO-C-ANCA vasculitis, an unusual IIF and ELISA pairing. The patient presented with lower extremity edema and without nasopharyngeal or pulmonary involvement, complicating the clinical picture for an ANCA-vasculitis. Given other published reports and studies pertaining to this unique finding of C-ANCA with anti-MPO antibodies, we emphasize the importance of using caution while interpreting ANCA results. The diagnosis of vasculitis should be comprehensive and should not solely rely on IIF or physical exam findings.