Serum cystatin C-based equation compared to serum creatinine-based equations for estimation of glomerular filtration rate in patients with chronic kidney disease.

Estimation of the glomerular filtration rate (GFR) is essential for the evaluation of patients with chronic kidney disease (CKD). The Cockcroft-Gault (CG) and modification of diet in renal disease (MDRD) formulas are serum creatinine-based equations, and the most widely used tests for renal function. Recently, serum cystatin C-based equations were proposed as markers for estimation of GFR. The present study compares our serum cystatin C-based equation (cystatin C formula) and serum creatinine-based equations for a large group of patients with CKD. In this study, 592 adult patients with CKD were enrolled. In each patient, serum creatinine was determined and creatinine clearance was calculated using the CG and MDRD formulas. The serum cystatin C was determined by an immunonephelometric method and our own cystatin C formula (GFR = 90.63 x cystatin C-1.192) for estimation of GFR was developed. GFR was measured using 51CrEDTA clearance, and the correlation, accuracy, bias and precision were determined. Ability to correctly estimate the patient's GFR with different equations compared to gold standard below and above 60 ml/min/1.73 m2; was analyzed. The mean 51CrEDTA clearance was 47 ml/min/1.73 m2, the mean serum creatinine was 269 micromol/l and the mean serum cystatin C was 2.68 mg/l. Statistically significant correlation between 51CrEDTA clearance with the CG (r = 0.861) and MDRD (r = 0.909) formulas and the cystatin C formula (r = 0.899) was found. The receiver operating characteristic (ROC) curve analysis (cut-off for GFR 60 ml/min/1.73 m2) showed that the cystatin C formula had a significantly higher diagnostic accuracy than the CG formula (p < 0.003). All equations underestimated the measured GFR and lacked precision. Analysis of ability to correctly predict the patient's GFR below or above 60/ml/min/1.73 m2 showed a higher prediction for the cystatin C formula than the MDRD formula (91.6 versus 84.1%, p < 0.0005) and a higher prediction trend than the CG formula (91.6 versus 88.3%, p = 0.078). Our results indicate that serum cystatin C-based equation is a reliable marker of GFR with a very high diagnostic accuracy and ability to predict patients with CKD and GFR under 60/ml/min/1.73 m2.