Free tool
eGFR after a creatinine method change
Enter one creatinine result and the difference between the old and new method to see how far eGFR and the KDIGO category move when only the method has changed.
How the answer is worked out
eGFR is calculated from creatinine, age and sex. When a laboratory changes creatinine method, the same sample can read higher or lower, and every eGFR moves with it. The calculator applies the method difference to your creatinine, then calculates eGFR both ways with the same equation, so the whole shift comes from the method.
The equations are CKD-EPI 2021, without race [Inker et al., 2021], and the European Kidney Function Consortium (EKFC) equation as tabulated by [Zhao et al., 2023]. Creatinine converts at 88.4 µmol/L per mg/dL. Categories are the KDIGO GFR categories G1 to G5, on the eGFR rounded to a whole number.
Where the method differences come from
The ready-made method changes come from published data: mean biases of creatinine peer groups in a provincial external quality assessment programme after IDMS standardisation [Lee et al., 2017], biases of three enzymatic platforms on commutable EQA samples [van der Hagen et al., 2021], and a regression line between two chemistry analysers on patient samples [Bush et al., 2020]. This site does not name analyser makers; the cited papers do. Your own comparison of the old and new method on shared samples is the better input.
How large is large?
eGFR varies from test to test even in a stable patient. A systematic review put the minimum conservative reference change value for eGFR at ±12.5% [Thöni et al., 2022]. The calculator shows the method shift as a share of that: a method change that uses up most of it makes a real decline harder to see, and can move a patient across a category line on its own.
For the chemistry behind the two creatinine methods, see Jaffe or enzymatic creatinine. To test whether a change between two of a patient's results is real, use Is this change real?
What it does not do
It applies one average line to one result; individual samples scatter around that line, and interferences differ between methods. It is for adults only, and it is not a diagnostic device.
The guide to eGFR after a creatinine bias follows a method bias through the equation and shows why the category lines sit where eGFR is most sensitive to it.
Sources
- Inker LA, Eneanya ND, Coresh J, et al. (2021). New creatinine- and cystatin C-based equations to estimate GFR without race. New England Journal of Medicine. pmc.ncbi.nlm.nih.gov/articles/PMC8822996/ (opens in a new tab)
- Zhao L, Li H, Liu X, et al. (2023). Validation of the EKFC equation for glomerular filtration rate estimation and comparison with the Asian-modified CKD-EPI equation in Chinese chronic kidney disease patients in an external study. Renal Failure. pmc.ncbi.nlm.nih.gov/articles/PMC9848359/ (opens in a new tab)
- Thöni S, Keller F, Denicolò S, et al. (2022). Biological variation and reference change value of the estimated glomerular filtration rate in humans: a systematic review and meta-analysis. Frontiers in Medicine. pmc.ncbi.nlm.nih.gov/articles/PMC9583397/ (opens in a new tab)
- Lee E, Collier CP, White CA (2017). Creatinine assay attainment of analytical performance goals following implementation of IDMS standardization. Canadian Journal of Kidney Health and Disease. pmc.ncbi.nlm.nih.gov/articles/PMC5347424/ (opens in a new tab)
- van der Hagen EAE, et al. (2021). Feasibility for aggregation of commutable external quality assessment results to evaluate metrological traceability and agreement among results. Clinical Chemistry and Laboratory Medicine. doi.org/10.1515/cclm-2020-0736 (opens in a new tab)
- Bush V, Smola C, Schmitt P (2020). Practical Laboratory Medicine. pmc.ncbi.nlm.nih.gov/articles/PMC6909053/ (opens in a new tab)