Urinary Retinol-Binding Protein 4 is Associated With Renal Function and Rapid Renal Function Decline in Kidney Transplant Recipients

•Urinary RBP4 levels are associated with renal function in kidney transplants.•Urinary RBP4 levels might be used to predict rapid renal function decline in kidney transplants. Urinary retinol-binding protein 4 (RBP4) has been known as a biomarker of chronic kidney disease. In this study, we evaluated the association of urinary RBP4 with renal function and progression of renal function in kidney transplant recipients (KTRs). A total 50 KTRs were included in this study. Proteomic analysis with liquid chromatography-mass spectrometry and tandem mass spectrometry was performed to discover potential urinary biomarkers. Several urinary proteins including RBP4 were identified and then validated by enzyme-linked immunosorbent assay. Rapid renal function decline was defined as estimated glomerular filtration rate (eGFR) decline of >3 mL/min/1.73 m2/year or initiation of dialysis, and 19 (38%) were included in rapid renal function decline group. Urinary RBP4/creatinine was inversely correlated with allograft function (r = –0.54, P < .001 with eGFR, and r = 0.49, P < .001 with serum creatinine, respectively). Urinary RBP4/creatinine was higher in rapid renal function decline group than in stable renal function group (184.9 ± 156.7 vs 83.1 ± 99.9, P = .017). Log-transformed urinary RBP4/creatinine was significantly associated with rapid renal function decline in univariate logistic regression analysis (Odds ratio [OR] 7.59, confidence interval [CI] 2.04-36.70, P = .005). In multivariate logistic regression adjusted with recipient age and sex, donor age, number of HLA mismatch, and acute rejection episode, urinary RBP4/creatinine remained a significant factor for rapid renal function decline (OR 9.43, CI 1.99-65.65, P = .010). Receiver operating characteristic analysis showed that the area under the curve of urinary RBP4/creatinine was 0.747 (CI 0.608-0.886, P < .001) for rapid renal function decline. Urinary RBP4 levels are associated with renal function and might be used to predict rapid renal function decline in KTRs.