Biological Variation of Creatinine, Cystatin C, and eGFR over 24 Hours

BACKGROUND: Estimated glomerular filtration rate (eGFR) is widely used in clinical practice. This study assessed the within-subject biological variation (CV I ) of different eGFR equations in people with chronic kidney disease (CKD) and people without CKD. The aims of this study were ( a ) to determine the 24-h biological variation profiles of creatinine, cystatin C, and eGFR and ( b ) to determine whether CV I of creatinine, cystatin C, and eGFR changes on deterioration of glomerular filtration. METHODS: Hourly blood samples were analyzed from 37 individuals (17 without CKD, 20 with CKD) during 24 h. Creatinine (enzymatic method) and cystatin C were measured using a Cobas 8000 (Roche Diagnostics). eGFR was estimated using the Modification of Diet in Renal Disease and the Chronic Kidney Disease Epidemiology Collaboration based on creatinine and/or cystatin C. Plasma samples were stored at −80 °C before analysis. Outlier and homogeneity analyses were checked before performing a nested ANOVA to determine biological variation. RESULTS: CV I of creatinine was higher in people without CKD than in those with CKD (6.4% vs 2.5%) owing primarily to the more profound effect of meat consumption on creatinine variability in individuals with lower baseline creatinine concentrations. Unlike creatinine, cystatin C concentrations were unaffected by meat consumption. Cystatin C showed some diurnal rhythmic variation and less in people with CKD. Reference change values (RCVs) of all eGFR equations were within 13% to 20% in both study groups. CONCLUSIONS: Despite differences in CV I of creatinine, the CV I and RCV of the eGFR equations were relatively similar for people with or without CKD.