In Reply

We would like to thank Prof. Rothenbacher and Prof. Koenig for their letter in response to our article. Indeed, our concern was to avoid overestimating the prevalence of patients with a reduced glomerular filtration rate. For this reason, we used the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation in combination with cystatin C levels.

With regard to Dr. Schneider’s argument that the calculation model used does not adequately take into account the body composition of the population under investigation, it should be noted that exactly this issue was addressed by including cystatin C levels in addition to serum creatinine levels. In line with the considerations mentioned above, it seems reasonable to call for an increased use of cystatin C testing in clinical routine, particularly in view of the pharmacological aspects.

From a historical perspective, it is understandable that the glomerular filtration rate was based on a standard body surface of 1.73 m², as this was the average body surface of 25-year-old men and women in the US in 1927 (1). However, as outdated as it may appear, the actual figure for the standard value is irrelevant, as it is arbitrarily chosen. But we think it is important that epidemiological studies continue to use the internationally agreed standard, for the very reason that, otherwise, the range of normal values of the estimated glomerular filtration rate would need to be defined for each population individually. However, on the level of managing individual patients, it may make sense to use a non-standardized value, accounting for height, weight and muscle mass.