RBP4 and chronic kidney disease: Thus, levels of α-1-microglobulin/bikunin precursor, beta-2-microglobulin, complement factor D, cystatin-C, and retinol-binding protein 4 in HDL were strongly correlated with levels of eGFR (supplemental Table S4) and steadily increased as CKD became more severe (supplemental Fig. S1).