**3. Indexing GFR for BSA**

The GFR of a healthy person can vary from 1 ml/min for neonates to 200 ml/min for large adults. White et al. stated that this makes the interpretation of a GFR measurement not easy for the physician unless the physician is familiar with the expected normal value for the particular patient. (White & Strydom, 1991) Therefore, it would be worth considering to normalize GFR in such a way that the influence of patient variables is minimal.

In 1928, renal function was for the first time corrected for BSA by McIntosh et al. (McIntosh et al., 1928) McIntosh et al. built their indexation theory on the experience of the research of Taylor et al., which assumed a correlation between urea excretion and the weight of the kidneys in rabbits. Taylor et al. also showed that there exists a better correlation between kidney weight and BSA, than between kidney weight and animal's weight. (Taylor et al., 1923) When McIntosh et al. on their turn corrected urea clearances of 18 adults and 8 children for BSA, the data from the small children yielded the same normal values as for adults. In the footsteps of Taylor et al., MacKay illustrated a direct correlation between BSA and kidney weight and between BSA and urea excretion in humans. (MacKay, 1932). Based on these observations, the indexation of GFR for BSA became standard in the medical community.

McIntosh et al. also introduced the use of the reference surface area of 1.73m², which was the average calculated BSA of 25 year old Americans at that time. The value of 1.73m² has served the physiological community well for nearly 80 years, but is clearly no longer applicable to modern Western populations, as has been shown by Heaf et al. (Heaf, 2007) A value of 1.95m² would probably be more appropriate for the average BSA of today's 25 year old adults in America. Switching from 1.73m² to 1.95m² has severe repercussions for the current classification system for Chronic Kidney Disease, which is based on fixed limits of 15, 30, 60 and 90 ml/min/1.73m². The importance of 1.73m² or 1.95m² is not the value as such, but the fact that it serves as a reference point. Therefore, there is no need to change the reference value.

Recently, Delanaye et al. recalculated Taylor's correlation and noted that the correlation between BSA and kidney weight was not different from that between kidney weight and body weight. (Delanaye et al., 2009a) This indicates that the BSA-indexation theory of McIntosh et al. was based on false assumptions.
