Serum cystatin C as a marker of the renal function

The protease inhibitor cystatin C is a non-glycosylated low molecular weigh t protein (M-r = 133 59) which is produced by all nucleated cells at a cons tant rate, freely filtered by the renal glomeruli, and catabolized in the t ubuli. The aim of the study was to elucidate the applicability of serum cys tatin C as a marker of glomerular filtration rate (GFR) in patients with va rious kidney diseases with a wide range of renal function and in dialysis p atients. Seventy-six patients with various kidney diseases (aged 20 to 79 y ears) and 61 dialysis patients (aged 21 to 82 years) were included. Serum c ystatin C was measured by automated particle-enhanced immunoturbidimetry, s erum and urine creatinine by an enzymatic method, and GFR by Tc-99m-DTPA-cl earance using a single plasma sample method. Serum cystatin C in patients w ith various kidney diseases was 1.90 +/- 0.98 mg/L (mean +/- SD) and in dia lysis patients 7.14 +/- 1.91 mg/L. In the non-dialysis patients a linear re lationship was found between Tc-99m-DTPA-clearance and 1/serum cystatin C ( r = 0.91, p-value < 0.0001), 1/serum creatinine (r = 0.89, p-value < 0.0001 ), and creatinine-clearance (r = 0.88, p-value < 0.0001). Comparison of the non-parametric ROC plots for serum cystatin C (area under the curve (AUC) = 0.9665; SE = 0.0169), serum creatinine (AUC = 0.9554; SE = 0.0205), and c reatinine-clearance (AUC = 0.9731; SE = 0.0160) revealed no significant dif ferences (p-values: 0.50, 0.78, and 0.49). In conclusion, cystatin C may be a likewise good marker of the GFR as serum creatinine and creatinine-clear ance, cystatin C having the advantage being independent of gender and muscl e mass.