title: Cystatin C as a filtration marker--haemodialysis patients expose its strengths and limitations.
abstract: OBJECTIVE
  The reliability of serum cystatin C (s-Cys) as a filtration marker depends on the intra- and inter-individual variation and influence of non-renal factors of its production rate (Cys(pr)), non-renal clearance (CL(nr)) and sieving coefficient (S). Haemodialysis patients with no residual renal function would be the best population in which to investigate these variables, which otherwise require reliable GFR measurements.
  
  
  METHODS
  Seventy-nine haemodialysis (HD) patients with negligible residual renal function (Group 1) were investigated and compared with 55 HD patients with varying degrees of residual renal function (Group 2) and 923 non-dialysis patients (Group 3). The equation eGFR = Cys(pr)/s-Cys-CL(nr) was used to analyse the turnover and variation of cystatin C.
  
  
  RESULTS
  A formula for estimating GFR, eGFR = 99/s-Cys-14.1, calculated from Group 3, was shown to fit the HD patients. The measured s-Cys in Group 1 was 6.9+/-0.9 and 6.4+/-1.1 mg/L in Group 2. The calculated 95% confidence interval of eGFR of +/-(30-40) % increased sharply below GFR 20 mL/min/1.73 m(2), which means that s-Cys cannot be used for calculating low GFR, including the residual GFR of dialysis patients.
  
  
  CONCLUSIONS
  Nicotine users in Group 1 had significantly higher s-Cys than non-users (7.5+/-0.9 mg/L compared to 6.7+/-0.8; p = 0.0008), which may be a factor to include in the eGFR formulae. However, s-Cys was independent of non-renal factors such as sex, age, LBM, body weight, malnutrition and CRP and also of changes in CRP.
authors: Department of Medicine, Orebro University Hospital, Orebro, Sweden. per.sjostrom@orebroll.se
authors: Per A
authors: PA
authors: Sjöström
authors: Ian L
authors: IL
authors: Jones
authors: Martin A
authors: MA
authors: Tidman
mesh_terms: D000368:Aged; D015415:Biomarkers / Q000097:blood; D001835:Body Weight; D002097:C-Reactive Protein / Q000378:metabolism; D055316:Cystatin C / Q000097:blood*; D003710:Demography; D005260:Female; D005919:Glomerular Filtration Rate; D006801:Humans; D007676:Kidney Failure, Chronic / Q000097:blood / Q000503:physiopathology; D007677:Kidney Function Tests; D008297:Male; D008875:Middle Aged; D006435:Renal Dialysis
chemical_list: D015415:Biomarkers; D055316:Cystatin C; D002097:C-Reactive Protein