title: Serum cystatin C is not a better marker of creatinine or digoxin clearance than serum creatinine.
abstract: OBJECTIVE
  To assess whether cystatin C, a new serum marker of renal function, is a better index of creatinine or digoxin clearance than serum creatinine in older people.
  
  
  METHODS
  Twenty-two volunteers over the age of 65 years (mean 73 +/- 5) were recruited from a healthy elderly volunteer database. None of the volunteers was taking digoxin or other medication known to interfere with digoxin kinetics or assay. Digoxin was infused at a dose of 7-10 microg kg(-1) and blood samples were taken over the following 48 h and assayed for serum digoxin. Serum cystatin C, creatinine and creatinine clearance were measured and a calculated creatinine clearance was estimated using the Cockcroft Gault formula. Digoxin clearance was calculated using a pharmacokinetic software package. All values were log transformed to normalize their distribution.
  
  
  RESULTS
  Of the 22 volunteers enrolled into the study, 18 completed the study. Serum cystatin C ranged between 0.72 and 1.89 mg l(-1) and serum creatinine ranged from 69.6 to 153.9 micromol l(-1). Measured creatinine clearance ranged from 38 to 123 ml min(-1) and calculated creatinine clearance from 29.5 to 88.0 ml min(-1). Digoxin clearance ranged from 51.0 to 103.5 ml min(-1). Cystatin C correlated extremely well with creatinine (r=0.93, P<0.001, 95% CI 0.82, 0.97) and with creatinine clearance (r=0.67, P=0.002, 95% CI 0.3, 0.87). Neither serum cystatin C nor serum creatinine correlated with digoxin clearance (r=0.25, P=0.31, 95% CI -0.25, 0.64 and r=0.44, P=0.068, 95% CI -0.03, 0.75, respectively). Measured creatinine clearance, however, did correlate well with digoxin clearance (r=0.55, P=0.018, 95% CI 0.11, 0.81).
  
  
  CONCLUSIONS
  Serum cystatin C and serum creatinine show very similar correlations with creatinine and digoxin clearances. Serum cystatin C does not offer any advantages in this respect. It remains to be seen whether cystatin C offers any advantage over creatinine in elderly people in other respects.
authors.affiliation: Clinical Age Research Unit, Department of Healthcare of the Elderly, Guys, Kings and St Thomas' School of Medicine, Denmark Hill Campus, Kings College London, London SE5 9PJ. oriordanlamb@ukonline.co.uk
authors.forename: S
authors.initials: S
authors.lastname: O'Riordan
authors.forename: E
authors.initials: E
authors.lastname: Ouldred
authors.forename: S
authors.initials: S
authors.lastname: Brice
authors.forename: S H D
authors.initials: SH
authors.lastname: Jackson
authors.forename: C G
authors.initials: CG
authors.lastname: Swift
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D015415:Biomarkers / Q000097:blood; D003404:Creatinine / Q000097:blood*; D055316:Cystatin C; D015891:Cystatins / Q000097:blood*; D004077:Digoxin / Q000097:blood / Q000493:pharmacokinetics*; D005260:Female; D005919:Glomerular Filtration Rate / Q000502:physiology; D006801:Humans; D007262:Infusions, Intravenous; D008297:Male; D008657:Metabolic Clearance Rate
chemical_list: D015415:Biomarkers; C525030:CST3 protein, human; D055316:Cystatin C; D015891:Cystatins; D004077:Digoxin; D003404:Creatinine