MPO and microscopic polyangiitis: Approximately 65–75% of GPA patients are anti-proteinase 3 (PR3)-ANCA positive and 20–30% have anti-myeloperoxidase (MPO)-ANCA while 20–30% of patients with MPA express anti-PR3 (αPR3)-ANCA and 55–65% are anti-MPO (αMPO)-ANCA positive [1].