MPO and rapidly progressive glomerulonephritis: Of the 9 MPO-ANCA-positive patients who developed MPA, 7 were diagnosed based on the pathological confirmation and systemic manifestations (kidney biopsy in rapidly progressive glomerulonephritis, n = 4; skin biopsy in purpuric rash, n = 2; gastrointestinal mucosa biopsy in gastrointestinal bleeding, n = 1), and 2 were diagnosed on the basis of surrogate markers for renal vasculitis plus diffuse alveolar haemorrhage [19] (S1 Table).