Microscopic polyangiitis is an ill-defined autoimmune disease characterized by a systemic, pauci-immune, necrotizing, small-vessel vasculitis without clinical or pathological evidence of necrotizing granulomatous inflammation. It is type III Rapidly progressive glomerulonephritis (RPGN). Around 90% patient s will have P-ANCA positive. Ref: Emil A. Tanagho, Jack W. McAninch (2004), Chapter 31, "Diagnosis of Medical Renal Diseases", In the book, "Smith's General Urology", USA, Page 523; Robbin's Basic Pathology, 7th Edition, Page 977