MPO and Nephropathy: Additionally, the levels of anti-myeloperoxidase (anti-MPO) and anti-proteinase 3 (anti-PR3) antibodies are useful to diagnose antineutrophil cytoplasmic antibodies associated with systemic vasculitis and related kidney damage (Beck et al., 2009; McAdoo & Pusey, 2017; Yang et al., 2007).