MPO and anti-neutrophil cytoplasmic antibody-associated vasculitis: According to the revised Chapel Hill Consensus Conference Nomenclature of vasculitides, AAV is defined as a necrotising vasculitis with few or no immune deposits, predominantly affecting small blood vessels (i.e. capillaries, venules, and arterioles), associated with the presence of circulating autoantibodies (ANCA) that are usually directed against myeloperoxidase (MPO) or proteinase 3 (PR3).