MPO and necrotizing vasculitis: The diagnosis of AAV is based on the combined evaluation of clinical symptoms, positive ANCA serology, and/or histological evidence of otherwise unexplained necrotizing vasculitis and/or granulomatous destructive parenchymal inflammation.[5–7] ANCAs are considered one of the most important serological biomarkers for AAV.[6] These antibodies react against cytoplasmic antigens expressed in the primary granules of neutrophils and the lysosomes of monocytes, particularly against neutrophil cytoplasmic components, such as proteinase 3 (PR3) and myeloperoxidase (MPO).[8]