A positive C-ANCA or PR3-ANCA is reported in 80–90% of patients with active GPA and approximately 50% of patients with inactive disease; whilst a P-ANCA/MPO-ANCA has been reported in 3.6 to 15% of patients in cohorts with varied disease activity [10, 15, 16]. This evidence concerns the gene PRTN3 and granulomatosis with polyangiitis.