MPO and microscopic polyangiitis: Systemic small vessel vasculitis with renal involvement is mostly associated with an anti-neutrophil cytoplasmic antibody (ANCA) directed at leukocyte proteinase 3 (PR3-ANCA) and myeloperoxidase (MPO-ANCA), including MPA, GPA, and eosinophilic granulomatosis with polyangiitis (Churg–Strauss).