The authors reply
We agree with Rosansky and Thamer1 that creatinine methodology is a critical component of glomerular filtration rate estimation. We can reconfirm that in our study, we used the same internationally standardized enzymatic creatinine assay throughout the study.2 Our study was not really a longitudinal study in the sense implied by Rosansky and Thamer. We studied biological variation of the estimated and measured glomerular filtration rates over a period of 4 weeks in patients with stable chronic kidney disease.
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