title: The impact of interlaboratory differences in cystatin C assay measurement on glomerular filtration rate estimation.
abstract: OBJECTIVE
  Cystatin C (CysC) is a promising marker of GFR. Several equations have been derived to estimate GFR from its serum concentration. Heterogeneity in the performance of these equations exists in validation studies even when the same CysC assay from the same manufacturer is utilized. This study was designed to examine the differences in CysC and GFR estimation (eGFR) using Siemens' nephelometric immunoassay and the Mayo Clinic equation. The ability of the eGFRs to predict measured GFR was also examined.
  
  
  METHODS
  Ninety-seven split samples were sent to laboratories at Children's Hospital of Eastern Ontario (CHEO) in Ottawa, Canada, and at the Mayo Clinic in Rochester, Minnesota.
  
  
  RESULTS
  The mean CHEO CysC was 0.17 mg/L (10%) lower than the mean Mayo Clinic CysC. Using the Mayo Clinic equation, the mean eGFR difference was 7.2 ml/min per 1.73 m(2) (15%). Approximately 36% of the results agreed within 10%, while 13% were discordant by greater than 30%. Larger absolute differences in mean eGFR between the two laboratories were found in the subgroup with CysC less than 1.41 mg/L as compared with the subgroup greater than 1.41 mg/L (9.5 versus 5.0 ml/min per 1.73 m(2)). Correction of CHEO values to the Mayo Clinic did not improve GFR estimation.
  
  
  CONCLUSIONS
  Significant differences in CysC measurement exist between laboratories using the same assay by the same manufacturer and these lead to clinically relevant differences in GFR estimation. This interlaboratory variability needs to be recognized when interpreting and comparing CysC and eGFR results.
authors.affiliation: Division of Nephrology, Department of Medicine, Queen’s University, Kingston, Ontario, Canada. cw38@queensu.ca
authors.forename: Christine A
authors.initials: CA
authors.lastname: White
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Rule
authors.forename: Christine P
authors.initials: CP
authors.lastname: Collier
authors.forename: Ayub
authors.initials: A
authors.lastname: Akbari
authors.forename: John C
authors.initials: JC
authors.lastname: Lieske
authors.forename: Nathalie
authors.initials: N
authors.lastname: Lepage
authors.forename: Steve
authors.initials: S
authors.lastname: Doucette
authors.forename: Greg A
authors.initials: GA
authors.lastname: Knoll
mesh_terms: D002138:Calibration; D055316:Cystatin C / Q000097:blood*; D005919:Glomerular Filtration Rate; D006801:Humans; D007753:Laboratories; D016014:Linear Models
chemical_list: D055316:Cystatin C