title: Analytical study of three cystatin C assays and their impact on cystatin C-based GFR-prediction equations.
abstract: BACKGROUND
  Cystatin C-based equations are used to estimate GFR. However, three cystatin C immunoassays are on the market. Difference in cystatin C assays could have strong consequences on the accuracy and precision of cystatin C-based equations. We have performed an analytical study of these three assays and studied potential differences between assays on the precision of cystatin C-based equations.
  
  
  METHODS
  We have studied imprecision, recovery, linearity and interferences of the three immunoassays (nephelometric assay from Siemens and turbidimetric assays from Dako and Gentian). The impact of differences in cystatin C assays has been studied for the equations published by Levey (Siemens assay) and Grubb (Dako assay).
  
  
  RESULTS
  Analytical performance of the Dako assay is slightly less high. For cystatin C values below 2.5 mg/L, no statistical difference is found between results given by the Dako and the Gentian assays. So, both assays can be used in the Grubb equation. Cystatin C results are different with the Siemens assay. The Levey equation, built with the Siemens assay, can only be used with cystatin C values measured with this assay. Using the Dako or Gentian assay results in the Levey equation can lead to differences in estimating GFR up to 6 mL/min/1.73 m2. Differences can reach 9.5 mL/min/1.73 m2 if the Siemens assay is used in the Grubb equation.
  
  
  CONCLUSIONS
  The Siemens and Gentian assays seem analytically more valid than the Dako assay for cystatin C determination. Differences in cystatin C assays can lead to significant differences in cystatin C-based equations. However, these differences seem less important than the differences observed with creatinine and creatinine-based equations.
authors.affiliation: Department of Nephrology-Dialysis, University of Liège, CHU Sart Tilman, 4000 Liège, Belgium. pierre_delanaye@yahoo.fr
authors.forename: Pierre
authors.initials: P
authors.lastname: Delanaye
authors.forename: Laurence
authors.initials: L
authors.lastname: Pieroni
authors.forename: Christelle
authors.initials: C
authors.lastname: Abshoff
authors.forename: Laurence
authors.initials: L
authors.lastname: Lutteri
authors.forename: Jean-Paul
authors.initials: JP
authors.lastname: Chapelle
authors.forename: Jean-Marie
authors.initials: JM
authors.lastname: Krzesinski
authors.forename: Bernard
authors.initials: B
authors.lastname: Hainque
authors.forename: Etienne
authors.initials: E
authors.lastname: Cavalier
mesh_terms: D000465:Algorithms; D002138:Calibration; D003401:Creatine / Q000097:blood; D055316:Cystatin C / Q000032:analysis*; D005919:Glomerular Filtration Rate / Q000502:physiology*; D006454:Hemoglobins / Q000032:analysis; D006801:Humans; D007118:Immunoassay; D009391:Nephelometry and Turbidimetry; D011237:Predictive Value of Tests; D011933:Reagent Kits, Diagnostic; D015203:Reproducibility of Results
chemical_list: D055316:Cystatin C; D006454:Hemoglobins; D011933:Reagent Kits, Diagnostic; D003401:Creatine