title: Stability of creatinine and cystatin C in whole blood.
abstract: BACKGROUND
  As yet little is known about the effect of delayed separation of whole blood stored at room temperature on the stability of the kidney function markers creatinine and cystatin C.
  
  
  METHODS
  We used plasma samples of 45 patients with a wide range of creatinine and cystatin C concentration. Samples were sent by post as whole blood, and differences in creatinine and cystatin C concentrations when measured (by enzymatic assay and PETIA, respectively) in plasma separated shortly after blood withdrawal or in plasma obtained after delayed separation at 24, 48 and 72 h. Intra- and inter-assay variability was assessed and total change limit was calculated to assess analyte stability.
  
  
  RESULTS
  Total change limit was 3.3% for creatinine and 3.9% for cystatin C. In whole blood creatinine and cystatin C remained stable up to 48 h. Delayed separation of whole blood did not induce more variability in measured concentrations of both analytes. Glomerular filtration rate estimated with the CKD-EPI equations showed less than 3 mL/min/1.73 m² difference when using creatinine or cystatin C concentration measured in plasma separated up to 48 h after blood withdrawal compared to plasma separated shortly after blood withdrawal. The new CKD-EPI equation that uses creatinine as well as cystatin C to estimate GFR showed even at 72 h less than 3 mL/min/1.73 m² difference.
  
  
  CONCLUSIONS
  Creatinine and cystatin C remain stable in whole blood stored at room temperature up to 48 h before separation, and changes in these analytes during this time period do not affect variability and eGFR.
authors.affiliation: Division of Nephrology, Department of Internal Medicine, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands.
authors.forename: E M
authors.initials: EM
authors.lastname: Spithoven
authors.forename: S J L
authors.initials: SJ
authors.lastname: Bakker
authors.forename: J E
authors.initials: JE
authors.lastname: Kootstra-Ros
authors.forename: P E
authors.initials: PE
authors.lastname: de Jong
authors.forename: R T
authors.initials: RT
authors.lastname: Gansevoort
mesh_terms: D015415:Biomarkers / Q000097:blood; D001793:Blood Preservation; D003404:Creatinine / Q000097:blood*; D055316:Cystatin C / Q000097:blood*; D057075:Enzyme Assays; D005919:Glomerular Filtration Rate; D006801:Humans; D057230:Limit of Detection; D015233:Models, Statistical; D055550:Protein Stability; D051436:Renal Insufficiency, Chronic / Q000097:blood* / Q000175:diagnosis; D015203:Reproducibility of Results; D013048:Specimen Handling / Q000592:standards
chemical_list: D015415:Biomarkers; D055316:Cystatin C; D003404:Creatinine
keywords: Creatinine; Cystatin C; Enzymatic assay; Stability; Whole blood