Cystatin C, beta 2-microglobulin, and retinol-binding protein as indicators of glomerular filtration rate: comparison with plasma creatinine.

BACKGROUND: The aim of this study was to assess the diagnostic accuracy of plasma levels of three low-molecular weight proteins cystatin C, beta 2-microglobulin, and retinol-binding protein, as indicators of impairment of glomerular filtration rate in comparison with plasma creatinine.
METHODS: Glomerular filtration rate (GFR) was measured in 110 patients (51 M and 59 F, aged 18--79 years); creatinine (Creat), cystatin C (Cys), beta 2-microglobulin (beta 2M), and retinol-binding protein (RBP) were determined on the same day. The correlation coefficients between the different markers and GFR were determined. Receiver-operating characteristics (ROC) analysis was performed to assess their diagnostic accuracy. Furthermore, the relationship between plasma levels of the examined markers of GFR and body weight, height, fat-free mass (FFM) and body cell mass (BCM) was determined. FFM and BCM were calculated by means of total body electrical impedance measurement.
RESULTS: Serum concentrations of Cys, beta 2M and RBP increase progressively with the reduction of GFR. The magnitude of the increase in blood levels of Creat and beta 2M was higher than the increase of Cys, and much more than that of RBP, in particular in patients with GFR<20 ml/min/1.73 m(2). The correlation coefficients between GFR and 1/plasma concentrations were 0.647 for Creat, 0.651 for Cys, 0.731 for beta 2M, and 0.406 for RBP. ROC analysis indicated that the accuracy of beta 2M and Cys, as indicators of different degrees of GFR impairment (<80, <60, and <40 ml/min per 1.73 m(2)), was similar to that of Creat, while the diagnostic accuracy of RBP resulted significantly lower than that of Creat for any level of GFR. In patients without renal failure (GFR>40 ml/min per 1.73 m(2)), plasma concentrations of Creat were positively correlated with body weight (P<0.01), height (P<0.01), FFM (P<0.001) and BCM (P<0.001). Serum concentrations of RBP resulted correlated with FFM (P<0.05) and BCM (P<0.05), while no correlation was found between anthropometric data and Cys and beta 2M.
CONCLUSION: Cystatin C and beta 2-microglobulin have a diagnostic accuracy very similar to that of creatinine, while retinol-binding protein is not an adequate marker of glomerular filtration.