MPO and granulomatosis with polyangiitis: Antineutrophil cytoplasmic antibodies (ANCAs) directed against proteinase-3 (PR3) or myeloperoxidase (MPO) are important diagnostic markers for small small-vessel vasculitic syndromes (i.e. Granulomatosis with polyangiitis, microscopic polyangiitis, Eosinophilic granulomatosis and polyangiitis), which are commonly referred to as ANCA-associated vasculitis (AAV) [1].