RBP4 and acute kidney injury: Serum RBP4 concentrations were also correlated with markers of renal failure, specifically increasing creatinine (r = 0.493, P < 0.001; Figure 3c), urea (r = 0.389, P < 0.001), cystatin C (r = 0.381, P < 0.001) and decreasing glomerular filtration rate (GFR; r = -0.526, P < 0.001).