Utility of cystatin C-derived equations for evaluation of residual renal function in peritoneal dialysis patients

Residual renal function (RRF) plays a key role in the follow-up of the patients undergoing chronic ambulatory peritoneal dialysis (CAPD). Available methods for measurement of RRF are cumbersome and rarely used, and alternatively, cystatin C-derived equations have been proposed.

Seventy-six adult CAPD patients were recruited. RRF was measured using the 24-hour urea-creatinine clearance method. Serum concentrations of cystatin C were determined. Glomerular filtration rate (GFR) was estimated using the two published equations of Hoek and colleagues, and Yang and colleagues. GFR was also estimated from serum creatinine using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formula.

Patients (age range 18-86 years) were on CAPD for a median of 24 months. Average serum concentrations of cystatin C was 5 ± 1.2 mg/L. Average RRF was 0.7 ± 1.6 mL/min/1.73 m(2). All three methods significantly overestimated the measured RRF values (Hoek: 4 ± 1.4; Yang: 4.5 ± 1.5; 7.4 ± 4 mL/min/1.73 m(2)). Based on Bland-Altman plots, all three methods yielded poor agreement with RRF (p < 0.001 for all tests), with Hoek's equation providing the narrowest limits of agreement [mean difference (limits of agreement): 3.4 (2.9-3.9)] and CKD-EPI the widest [6.7 (5.9-7.5)]. Although the Hoek's method outperformed CKD-EPI, the within 30 and 50% accuracy rates were unsatisfactory (10.5 and19.7 %, respectively).

Cystatin C-derived equations outperform the CKD-EPI formula in approximating the RRF values. Yet, these methods still significantly overestimate the measured RRF and their routine application in clinical practice is not advised.