Alternatively, it may in part also reflect misclassification in the medical community in which patients who test positive for myeloperoxidase antineutrophil cytoplasmic antibodies are classified as having microscopic polyangiitis; and only patients who test positive for proteinase 3 antineutrophil cytoplasmic antibodies are classified as having granulomatosis with polyangiitis. Here, MPO is linked to granulomatosis with polyangiitis.