This assay is useful for evaluating patients suspected of having Autoimmune vasculitis, both Wegener's granulomatosis and Microscopic Polyangiitis. Autoantibodies to PR3 ( c-ANCA) occur in patients with classical / limited end-organ involvement Wegener's granulomatosis. Antibodies to MPO (p-ANCA) occur predominantly in patients with Microscopic Polyangiitis.