MPO and necrotizing vasculitis: Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV), a group of necrotizing vasculitis predominantly affecting small vessels, can be classified as antibody-specific subtypes into myeloperoxidase-specific ANCA (MPO-ANCA) and proteinase 3 ANCA (PR3-ANCA)3.