**5. Criticism on indexing GFR for BSA**

Indexing GFR for BSA goes back to 1928 and it has become so conventional that BSAindexing can be considered as an icon in nephrology. Nevertheless, during the last ten years there is increasing criticism on BSA indexation. According to Tanner, the dispersion of differences between data of children and adults is not a very strong argument for BSA indexing. (Tanner, 1949) Neither is the argument that BSA indexation is necessary because everybody does it and in that way results become comparable. (Kronmal, 1993) BSA indexation is also seriously questioned in populations with unusual anthropometric data such as in children, in obese or lean persons. According to Bird et al. indexing GFR for BSA does not suit children because they naturally have a relatively high BSA simply because of their small size. (Bird et al., 2003) Delanaye et al. studied obese and anorectic patients. (Delanaye et al., 2005, 2009a, 2009b) In those patients, the consequences of indexing for BSA are much more important since the cGFR is influenced by weight-variation and may obscure variations in the absolute GFR. That is why Delanaye et al. recommend using the absolute

Is Body Surface Area the Appropriate Index for Glomerular Filtration Rate? 11

possible indexator. (Peters et al., 1994b) However, it intuitively seems better to correct GFR for body fluid, since one of the roles of the kidneys is to regulate body fluid composition. Different GFR-indexators in that area are total body water (Bird, 2003; McCance & Widdowson, 1952), plasma volume (Peters et al., 1994a) or extracellular volume (ECV) (Bird, 2003; Peters, 1992, 1994b, 2000; White & Strydom, 1991) In 1952, McCance et al claimed that total body water was the best variable to index the GFR for children. (McCance & Widdowson, 1952) But since total body water is complex to determine every time the GFR is measured, ECV became the most

In this section we compare indexing GFR for BSA and for the alternatives height and ECV. True mathematical evidence for normalizing a physical quantity by any index is wellknown. (Turner & Reilly, 1995) The uncorrected quantity should be a linear function of the indexator with zero intercept (Figure 3A). After indexation the relationship between the

considered alternative to index the GFR, especially in children.

**6.1 Comparison of indexing GFR for BSA, height and ECV**

A. B.

relationship indexed GFR-indexator.

**6.1.1 Indexing GFR for BSA** 

**6.1.2 Indexing GFR for height** 

indexed quantity and the indexator then completely disappears (Figure 3B).

Fig. 3. (A) Linear regression of GFR versus the indexator; (B) Disappearance of the

children was calculated with the ECV formula of Bird et al. (Bird, 2003)

mathematical requirement for an indexator is fulfilled.

Because of their rapidly increasing size and renal maturation, children may give insight into the properties of different normalization indexes. Publicly available data are used to test the mathematical requirements for the indexators BSA, height and ECV. The dataset contains data for healthy children (between 0 and 15 years) of absolute and BSA-corrected median GFR values (51Cr-EDTA), median heights and weights. (Pottel et al., 2010) ECV of the

When considering median absolute GFR values versus BSA, one may observe a linear relationship (y = 59.96x with R² = 0.96) (Figure 4A). When GFR is indexed for BSA, the relation GFR-BSA disappears once the kidneys reach maturity (Figure 4B). So, the

The most evoked factor to index GFR in obese patients is height. (Anastasio, 2000; Schmieder, 1995) Again, we studied the fundamental prerequisite relationship GFR-height and the lack of relationship between GFR indexed for height and height. When the

GFR values instead of the cGFR, especially in 'abnormal' body size populations. (Delanaye, 2009a, 2009b) Geddes et al. started their article with another interesting case, in which they show that indexing for BSA can lead to a different clinical decision especially in the overweight. (Geddes et al., 2008) The case described by Geddes et al. concerns a 54 year old obese man who wants to donate a kidney to his own son. Published International guidelines and UK guidelines recommend a minimum GFR of 80 ml/min/1.73m² and 75 ml/min/1.73m² respectively for a 55 year old kidney donor. Direct measurement of the kidney function of the man resulted in an absolute GFR of 87 ml/min and in a corrected cGFR of 77.9 ml/min/1.73m². It is clear that the difference between the absolute GFR and the cGFR is of major importance in this case. Above all, we may not forget that there are several formulas to estimate the BSA. In Table 5 we illustrate that using another BSA formula can influence the decision. Since the studied man is obese (BMI of 31.5), the formula of Livingston & Lee (Livingston & Lee, 2001), which results in a negative decision towards the kidney donation, should probably be preferred.


Table 5. BSA calculated with frequently used BSA formulas and comparative BSA indexed GFR values of a 54 year old man with a length of 165 cm, a weight of 86 kg and an absolute GFR of 87 ml/min.
