Association of the Red Blood Cell Distribution Width-to-Albumin Ratio with Renal Outcomes in Chinese Patients with IgA Nephropathy

Background: The red blood cell distribution width-to-albumin ratio (RAR) is a novel composite biomarker that integrates hematological, inflammatory, and nutritional information, but its association with renal outcomes in immunoglobulin A nephropathy (IgAN) remains unclear. This study aimed to evaluate the association between RAR and long-term renal outcomes in patients with biopsy-proven IgAN. Methods: This study was a retrospective analysis of an established observational cohort, including 1169 patients with biopsy-proven IgAN. RAR was analyzed both categorically, using a receiver operating characteristic (ROC)-derived cutoff, and continuously. Propensity score matching (PSM) was performed at a 1:1 ratio. Kaplan–Meier analysis and multivariable Cox proportional hazards models were used to evaluate the association between RAR and the composite renal endpoint. Restricted cubic spline (RCS) analysis was used to assess the shape of the association. Results: Before PSM, higher RAR was independently associated with the composite renal endpoint in the fully adjusted model (HR 1.880, 95% CI 1.222–2.893; p = 0.004). The association remained significant after PSM (HR 2.128, 95% CI 1.067–4.243; p = 0.032). When analyzed continuously, RAR was associated with a higher risk of the composite endpoint before PSM (HR 1.371, 95% CI 1.099–1.711; p = 0.005) and after PSM (HR 1.989, 95% CI 1.199–3.299; p = 0.008). RCS analyses showed significant overall associations before and after PSM, without significant evidence of nonlinearity. Conclusions: Higher RAR was independently associated with adverse renal outcomes in patients with IgAN. Further external validation and large multicenter prospective studies are needed to determine its clinical value.