First described in patients with necrotizing crescentic glomerulonephritis (NCGN) without immune deposits (pauci-immune), the clinical spectrum associated with anti-MPO includes also NCGN associated with systemic vasculitis, either Granulomatosis with polyangiitis (GPA) or a microscopic polyangiitis (MPA). Indeed, anti-MPO are detectable in 65% of patients with idiopathic NCGN, 45% of patients with MPA and 20% to 30% of patients with GPA. Additionally, anti-MPO are present in some 60% of patients with the eosinophilic granulomatosis with polyangiitis (EGPA). Antibodies to PR3 are highly sensitive (81%) and specific (97%) for GPA. The sensitivity is dependent on the phase and on the activity of the disease. Despite the strong association between PR3 antibodies and GPA, there is a small percentage of patients with microscopic polyangiitis and about 30% of EGPA patients who are PR3 antibodies positive. PR3 antibodies may also occur in 20% to 30% of patients with necrotizing glomerulonephritis with no obvious extrarenal manifestations of small vessel vasculitis.
Preparation of Patient: There is no special physical preparation required.