MPO and necrotizing vasculitis: In contrast, antineutrophil cytoplasmic antibody-associated vasculitis (AAV) is defined as a necrotizing vasculitis with few or no immune deposits that predominantly affects small vessels, associated with myeloperoxidase-directed antineutrophil cytoplasmic antibodies (MPO-ANCA) or proteinase 3-directed antineutrophil cytoplasmic antibodies (proteinase 3-ANCA) [3], with more than 75% of patients having kidney involvement [4].