title: Comparison of within- and between-subject variation of serum cystatin C and serum creatinine in children aged 2-13 years.
abstract: BACKGROUND
  Previously, data on both the within-subject (SD(I)) and the between-subject (SD(G)) variation of cystatin C in children has not been reported. Thus, this study aimed to determine this biological variation including analytical variation (SD(A)) of both cystatin C and creatinine to characterize the two analytes as renal function markers in children.
  
  
  METHODS
  On two consecutive days blood samples for duplicate analysis of cystatin C (nephelometric, Dade Behring) and creatinine (enzymatic, Roche) were obtained from 30 children (11 females and 19 males, mean age 8.3 range 2-13 years) referred for GFR measurements by (51)Cr-EDTA clearance. For determination of the between-subject variation only children with normal GFR (n=21) were included. Data were adjusted for the well known age-related increase in creatinine.
  
  
  RESULTS
  The results are given as coefficients of variation. The within-subject variations were identical for both analytes (6.4%). The between-subject variation was 11.1% for cystatin C and 28.4% for creatinine, though decreasing to 20.1% after adjusting for age. The analytical variation was 1.7% and 2.5% for cystatin C and creatinine, respectively. The index of individuality (IOI = SD(I)/SD(G)) was 0.65 for cystatin C and 0.25 for creatinine, though increasing to 0.36 after age-adjustment.
  
  
  CONCLUSIONS
  The within-subject variation was identical and low for cystatin C and creatinine suggesting that the two are equally suitable for serial monitoring of renal function in children. Based on the low IOI neither analyte, however, seems suitable as a screening marker of renal function in a healthy population of children using population-based reference intervals.
authors.affiliation: Department of Nuclear Medicine, Aalborg Hospital, Aarhus University Hospital, Aalborg, Denmark. tba@rn.dk
authors.forename: Trine B
authors.initials: TB
authors.lastname: Andersen
authors.forename: Erland J
authors.initials: EJ
authors.lastname: Erlandsen
authors.forename: Jørgen
authors.initials: J
authors.lastname: Frøkiaer
authors.forename: Anni
authors.initials: A
authors.lastname: Eskild-Jensen
authors.forename: Jens
authors.initials: J
authors.lastname: Brøchner-Mortensen
mesh_terms: D000293:Adolescent; D015415:Biomarkers / Q000097:blood*; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D003404:Creatinine / Q000097:blood*; D055316:Cystatin C / Q000097:blood*; D005260:Female; D005919:Glomerular Filtration Rate; D006801:Humans; D007677:Kidney Function Tests / Q000379:methods; D008297:Male; D012016:Reference Values; D015203:Reproducibility of Results
chemical_list: D015415:Biomarkers; D055316:Cystatin C; D003404:Creatinine