Is cystatin C a better marker than creatinine for evaluating residual renal function in patients on continuous ambulatory peritoneal dialysis?

BACKGROUND: Current clinical assessments of residual renal function (RRF) for continuous ambulatory peritoneal dialysis (CAPD) patients usually require 24 h of urine collection, which is sometimes difficult for patients and contributes to random errors. Objective. Our study aims to investigate whether serum cystatin C (CysC) can serve as a better marker of RRF than serum creatinine (Cr) in CAPD and to develop a formula to estimate RRF with CysC levels.

METHODS: One hundred and sixty CAPD patients from a single dialysis unit were randomly divided into modeling (n(1) = 120) and validation (n(2) = 40) groups. RRF was assessed as the average of the renal clearances of urea and creatinine. We then derived RRF formulas based on the CysC and Cr levels from the modeling group and validated them by comparison with a published CysC-based equation and Modification of Diet in Renal Disease formula.

RESULTS: CysC levels were inversely related to RRF, Kt/V(urea) and total weekly Ccr but were unrelated to age, gender, body mass index, diabetes or peritoneal clearance. The RRF formulas derived from CysC and Cr were (sinh(ln(6.736-0.566 CysC)))(2) and (sinh(ln(6.097-0.265 Cr)))(2), respectively. When applied to the validation group, the estimated RRF based on CysC (2.8 ± 1.2 mL/min/1.73 m(2)) was similar to that of on Cr (2.8 ± 1.3 mL/min/1.73 m(2)) and the measured RRF (2.9 ± 1.7 mL/min/1.73 m(2)). The CysC formula showed a small bias, with the best 30 and 50% accuracy and had a larger area under the curve and higher sensitivity and specificity when compared to the Cr formula and other formulas.

CONCLUSION: Serum CysC may be a good marker for the estimation of RRF in CAPD patients. The derived CysC formula may be used to reliably estimate RRF in CAPD patients without the need for collection of 24 h urine.