MPO and necrotizing vasculitis: Antineutrophil cytoplasmic autoantibody (ANCA)-associated systemic vasculitis a group of systemic necrotizing vasculitides, which often involve small vessels and lead to few or no immune deposits in affected organs.[1] ANCAs directed against proteinase-3 (PR3) or myeloperoxidase (MPO) are important diagnostic markers for ANCA-associated vasculitis (AAV).[2] ANCAs can also be present in cases of tumors, infections, and other autoimmune diseases.