RESEARCH ARTICLE Glomerular Filtration Rate i

For routine applications or for epidemiological studies, GFR values are estimated on the basis of individual serum creatinine measurements by means of regression formulae, e.g., the MDRD4 and CKD-EPI formulae [1-4]. These so-called eGFR formulae are derived from population-data describing the relations between GFR values obtained by a kinetic methods (mGFR) and the corresponding concen-trations of serum creatinine [5]. Unfortunately, these formula-derived GFR-values are problematic in individuals with normal or mildly im-paired renal function. Several reasons are responsible for this problem, such as, e.g., the natural variation of the original population data in combination with the confi-dence intervals increasing with decreasing creatinine concentrations, and the increasing impact of analytical uncertainties of creatinine measurements particularly in * Correspondence