MPO and anti-neutrophil cytoplasmic antibody-associated vasculitis: Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a small to medium vessel vasculitis characterized by disease relapses, increased risk of end-stage renal disease, and excess mortality.[1,2] Most AAV patients have circulating ANCA that target proteinase 3 (PR3) or myeloperoxidase (MPO) and are considered pathogenic.[3] ANCA testing has been a central component of AAV diagnosis since the 1980s,[4,5] but the measurement of ANCA titers after treatment has been a controversial practice.