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.
