Estimation of residual glomerular filtration rate in dialysis patients from the plasma cystatin C level

Background. Residual renal function influences morbidity, mortality and quality of life of chronic dialysis patients. Residual glomerular filtration rate (rGFR) is therefore an important parameter in the follow-up of these patients. Because rGFR is measured as the mean of creatinine and urea clearance, a complete 24 h urine collection is essential, but often very difficult to manage for these patients. Methods. We investigated if plasma cystatin C (cysC) could give a good estimate of rGFR in dialysis patients and compared it to the measured rGFR, as well as to the rGFR estimate obtained with the Modification of Diet in Renal Disease (MDRD) formula. A total of 465 patients were included in this study. CysC levels of 215 haemodialysis (HD) and 95 chronic ambulatory peritoneal dialysis (PD) patients were used to derive a formula for rGFR. This formula was tested in the validation group of 107 HD and 48 PD patients. Results. The cysC formula derived in the modelling group was rGFR = -0.70 + 22 x (1/cysC). The mean estimated rGFR obtained with this formula in the validation group was not significantly different from the mean measured rGFR: difference 0.19 ml/min/1.73 m(2), 95% confidence interval (CI) -2.37 to 2.75ml/min/1.73 m(2). The MDRD formula gave a larger difference from the mean measured rGFR (3.13 ml/min/1.73 m(2)) and a much wider 95% Cl (-1.29 to 7.55ml/min/1.73 m(2)). A separate model for HD and PD patients did not improve the estimation of rGFR. Conclusions. The cysC formula showed better accuracy and precision than the MDRD formula. Therefore the cysC formula and not the MDRD formula should be used to calculate rGFR in dialysis patients when no 24 h urine sample is available.