Proteinase 3 (PR3) is a multifunctional protein found in the azurophil (primary) granules of neutrophils, in the granules of monocytes, and in the cytoplasm of endothelial cells. Antibodies against PR3 are highly specific for WG. The diagnostic sensitivity of these AABs is dependent on the stage and activity of disease: roughly 50% in the inactive initial stage, roughly 60% in active mono- or oligosymptomatic forms (kidney or lung involvement), and virtually 100% in the active generalized phase. A positive PR3-ANCA result is highly specific and permits the definitive diagnosis of early and abortive forms of WG as well as a number of limited forms of WG, e.g., in patients with scleritis, episcleritis, subglottic stenosis, Tolosa-Hunt syndrome, facial paresis, cranial polyneuritis, peripheral neuropathy, secondary polychondritis, pulmonary hemorrhage, idio-pathic progressive necrotizing nephritis, and hemodialysis patients with renal failure of unclear origin (reviewed in ). PR3-ANCAs are also found at low frequencies in other vasculitic diseases associated with WG (e.g., microscopic polyangiitis, Churg-Strauss syndrome, classical panarteritis nodosa). In WG, PR3-ANCA titers correlate with disease activity, i.e., they decrease in remission (response to therapy) and increase when exacerbation is imminent. PR3-ANCA monitoring can therefore be used to ensure optimal patient management.
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