Comparative performance of serum cystatin-c versus serum creatinine in diabetic subjects

Objective: Serum cystatin-C, a protein with constant production rate, undergoes glomerular filtration. Cystatin-C is a candidate surrogate marker, allegedly superior to serum creatinine, for estimating glomerular filtration rate, due to its high correlation with absolute measurement of the latter. The aim of this study was to assess from intra- and inter-subject variability the performance of cystatin-C in a cohort of diabetic patients spanning a wide range of kidney function and to compare it to that of serum creatinine. Research design and methods: 98 consecutive diabetic in-patients (45 type 1 and 53 type 2) were included. Mean age was 53 15 years (10). Creatinine clearance was 98 ml/min (median; range: 16-244). We used the discriminant ratio (DR) methodology to compare the performance of serum cystatin-C vs. that of creatinine to segregate subjects according to their glomerular filtration rate. Results: Serum creatinine values on day 1 and 2 were 1.10 +/- 0.76 and 1.07 +/- 0.89 mg/dl and concentrations of cystatin-C were 1.10 +/- 0.60 and 1.06 +/- 0.63 mg/L. A close linear relationship was observed between means of duplicates for creatinine and cystatin-C (Pearson product-moment correlation 0.92). DR was obtained from the ratio of the underlying between-subject to the within-subject standard deviations. DR values were 5.23 for creatinine and 8.82 for cystatin-C (P < 0.0001), implying superior discriminating ability for cystatin-C. Once adjusted for attenuation, measured Pearson product-moment correlation rose from 0.92 to 0.97. The DR methodology allowed for deriving an unbiased linear regression equation between methods, with slope and intercept at 0.79 and 0.23, respectively. Conclusions: Serum cystatin-C better discriminates among a population of type 1 and 2 diabetic patients with regard to their estimated glomerular filtration rate when compared with conventional serum creatinine measurement.