Serum Metrnl as a potential biomarker for renal involvement in ANCA-associated vasculitis.

Anti-neutrophil cytoplasmic antibody-associated vasculitis (AAV) is a systemic autoimmune disease characterized by significant renal involvement, yet identifying novel biomarkers for renal complications remains a clinical priority. Metrnl is a recently identified immunomodulatory cytokine implicated in inflammation, but its specific role in AAV has historically been unknown. To address this, this study investigated serum Metrnl levels via ELISA in 37 patients with microscopic polyangiitis (MPA), 17 with granulomatosis with polyangiitis (GPA), and 30 healthy controls (HCs), analysing correlations with clinical parameters such as the Birmingham Vasculitis Activity Score (BVAS) and renal function indicators under false discovery rate (FDR) correction. The results demonstrated that serum Metrnl levels were significantly elevated in both MPA and GPA patients compared to HCs and exhibited a strong positive correlation with BVAS in both subgroups. Crucially, following FDR adjustment, Metrnl levels showed significant correlations with key markers of renal impairment, including creatinine, cystatin C, and estimated glomerular filtration rate (eGFR). Stratification of MPA patients based on renal function (eGFR cut-off: 60 ml/min/1.73 m²) further revealed substantially higher Metrnl levels in those with impaired renal function. Receiver operating characteristic curve analysis indicated superior diagnostic efficacy for Metrnl in identifying AAV with renal involvement [area under the curve (AUC) = 0.8150] compared to diagnosing AAV overall (AUC = 0.7214). Collectively, these findings provide the first evidence that serum Metrnl is elevated in AAV and associated with disease activity and renal dysfunction, suggesting that Metrnl warrants further investigation as a potential biomarker for renal involvement in AAV.