The Clinical Application of Urine Soluble CD163 in ANCA-Associated Vasculitis.

Background Up to 70% of patients with ANCA-associated vasculitis (AAV) develop glomerulonephritis, with 26% progressing to ESKD. Diagnostic-grade and noninvasive tools to detect active renal inflammation are needed. Urinary soluble CD163 (sCD163) is a promising biomarker of active renal vasculitis, but a diagnostic-grade assay, assessment of its utility in prospective diagnosis of renal vasculitis flares, and evaluation of its utility in proteinuric states are needed. Methods We assessed a diagnostic-grade urinary sCD163 assay in (1) a real-world cohort of 405 patients with AAV and 121 healthy and 488 non-AAV disease controls; (2) a prospective multicenter study of 84 patients with potential renal vasculitis flare; (3) a longitudinal multicenter cohort of 65 patients with podocytopathy; and (4) a cohort of 29 patients with AAV (with or without proteinuria) and 10 controls. Results We established a diagnostic reference range, with a cutoff of 250 ng/mmol for active renal vasculitis (area under the curve [AUC], 0.978). Using this cutoff, urinary sCD163 was elevated in renal vasculitis flare (AUC, 0.95) but remained low in flare mimics, such as nonvasculitic acute kidney injury. Urinary sCD163's specificity declined in AAV patients with nephrotic-range proteinuria and in primary podocytopathy, with 62% of nephrotic patients displaying a "positive" urinary sCD163. In AAV patients with significant proteinuria, urinary sCD163 normalization to total urine protein rather than creatinine provided the greatest clinical utility for diagnosing active renal vasculitis. Conclusions Urinary sCD163 is elevated in renal vasculitis flare and remains low in flare mimics. Nonspecific protein leakage in nephrotic syndrome elevates urinary sCD163 in the absence of glomerular macrophage infiltration, resulting in false-positive results; this can be corrected with urine protein normalization.