title: Validation of cystatin C-based equations for evaluating residual renal function in patients on continuous ambulatory peritoneal dialysis.
abstract: BACKGROUND
  Residual renal function needs to be assessed frequently in patients on continuous ambulatory peritoneal dialysis (CAPD). A commonly used method is to measure creatinine (Cr) and urea clearance in urine collected over 24 h, but collection can be cumbersome and difficult to manage. A faster, simpler alternative is to measure levels of cystatin C (CysC) in serum, but the accuracy and reliability of this method is controversial. Our study aims to validate published CysC-based equations for estimating residual renal function in patients on CAPD.
  
  
  METHODS
  Residual renal function was measured by calculating average clearance of urea and Cr in 24-h urine as well as by applying CysC- or Cr-based equations published by Hoek and Yang. We then compared the performance of the equations against the 24-h urine results.
  
  
  RESULTS
  In our sample of 255 patients ages 47.9 ± 15.6 years, the serum CysC level was 6.43 ± 1.13 mg/L. Serum CysC level was not significantly associated with age, gender, height, weight, body mass index, hemoglobin, intact parathyroid hormone, normalized protein catabolic rate or the presence of diabetes. In contrast, serum CysC levels did correlate with peritoneal clearance of CysC and with levels of prealbumin and high-sensitivity C-reactive protein. Residual renal function was 2.56 ± 2.07 mL/min/1.73 m 2 based on 24-h urine sampling, compared with estimates (mL/min/1.73 m 2 ) of 2.98 ± 0.66 for Hoek's equation, 2.03 ± 0.97 for Yang's CysC-based equation and 2.70 ± 1.30 for Yang's Cr-based equation. Accuracies within 30%/50% of measured residual renal function for the three equations were 29.02/48.24, 34.90/56.86 and 31.37/54.90.
  
  
  CONCLUSIONS
  The three equations for estimating residual renal function showed similar limits of agreement and differed significantly from the measured value. Published CysC-based equations do not appear to be particularly reliable for patients on CAPD. Further development and validation of CysC-based equations should take into account peritoneal clearance of CysC and other relevant factors.
authors.affiliation: Department of Nephrology, West China Hospital, Sichuan University, Chengdu, Sichuan, People's Republic of China.
authors.forename: Hui
authors.initials: H
authors.lastname: Zhong
authors.affiliation: Department of Epidemiology and Biostatistics, West China School of Public Health, Sichuan University, Chengdu, Sichuan, People's Republic of China.
authors.forename: Wei
authors.initials: W
authors.lastname: Zhang
authors.affiliation: Department of Nephrology, West China Hospital, Sichuan University, Chengdu, Sichuan, People's Republic of China.
authors.forename: Min
authors.initials: M
authors.lastname: Qin
authors.affiliation: Institute of Clinical Trials, West China Hospital, Sichuan University, Chengdu, Sichuan, People's Republic of China.
authors.forename: ZhongPing
authors.initials: Z
authors.lastname: Gou
authors.affiliation: Institute of Clinical Trials, West China Hospital, Sichuan University, Chengdu, Sichuan, People's Republic of China.
authors.forename: Ping
authors.initials: P
authors.lastname: Feng
mesh_terms: D000328:Adult; D000465:Algorithms; D015415:Biomarkers / Q000097:blood; D002097:C-Reactive Protein / Q000378:metabolism; D003404:Creatinine / Q000097:blood; D055316:Cystatin C / Q000097:blood*; D005260:Female; D005919:Glomerular Filtration Rate; D005921:Glomerulonephritis / Q000097:blood* / Q000175:diagnosis / Q000503:physiopathology / Q000628:therapy; D006801:Humans; D008297:Male; D008875:Middle Aged; D010531:Peritoneal Dialysis, Continuous Ambulatory; D015203:Reproducibility of Results; D014508:Urea / Q000097:blood
chemical_list: D015415:Biomarkers; D055316:Cystatin C; D014508:Urea; D002097:C-Reactive Protein; D003404:Creatinine
keywords: continuous ambulatory peritoneal dialysis; cystatin C; equation; peritoneal dialysis; residual renal function