**6. Are there alternatives to index GFR?**

One may wonder why indexing GFR for BSA is still routinely used, despite all the criticism. The fact that the frequently used estimated eGFR formulas as well as the fixed cut-off values of the current Chonic Kidney Disesase classification system are both expressed in ml/min/1.73m² might be a first and important reason why it is not obvious to stop indexing GFR for BSA. A second reason might be that indexing GFR for BSA has almost no consequences for normally built people. Questions about the effectiveness of indexing GFR for BSA only appear when 'abnormal' body size populations like children or obese patients are studied. Due to the limitations of correcting GFR for BSA in those specific populations a whole array of alternative variables for indexing GFR has been suggested.

The most evoked factor to index GFR in obese patients is height. Two studies have shown that correcting GFR for height is identical in obese and non-obese populations, whereas corrected GFR for BSA is inadequately lower in the obese population. (Anastasio et al., 2000; Schmieder et al., 1995) Delanaye et al. stated that since the range of height in the population is narrower than the range of weight (giants and dwarfs are less numerous than obese or anorectic) it is logical that indexing for height will decrease the dispersion of data in the adult population. (Delanaye, 2009a) Other height-dependent indexators that have been proposed to normalize the GFR are lean body mass (Hallynck et al., 1981; Kurtin, 1988), ideal weight (Walser, 1990) and even squared height. (Mitch & Walser, 2000) Also body weight has been indicated as a

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

**FORMULA BSA (m²) cGFR (ml/min/1.73m²)**

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

One may wonder why indexing GFR for BSA is still routinely used, despite all the criticism. The fact that the frequently used estimated eGFR formulas as well as the fixed cut-off values of the current Chonic Kidney Disesase classification system are both expressed in ml/min/1.73m² might be a first and important reason why it is not obvious to stop indexing GFR for BSA. A second reason might be that indexing GFR for BSA has almost no consequences for normally built people. Questions about the effectiveness of indexing GFR for BSA only appear when 'abnormal' body size populations like children or obese patients are studied. Due to the limitations of correcting GFR for BSA in those specific populations a

The most evoked factor to index GFR in obese patients is height. Two studies have shown that correcting GFR for height is identical in obese and non-obese populations, whereas corrected GFR for BSA is inadequately lower in the obese population. (Anastasio et al., 2000; Schmieder et al., 1995) Delanaye et al. stated that since the range of height in the population is narrower than the range of weight (giants and dwarfs are less numerous than obese or anorectic) it is logical that indexing for height will decrease the dispersion of data in the adult population. (Delanaye, 2009a) Other height-dependent indexators that have been proposed to normalize the GFR are lean body mass (Hallynck et al., 1981; Kurtin, 1988), ideal weight (Walser, 1990) and even squared height. (Mitch & Walser, 2000) Also body weight has been indicated as a

whole array of alternative variables for indexing GFR has been suggested.

**DuBois & DuBois** 1.93 78 **Boyd** 1.98 76 **Gehan & George** 2.01 75 **Haycock** 2.05 73 **Mosteller** 1.99 76 **Livingston & Lee** 2.09 72

the kidney donation, should probably be preferred.

**6. Are there alternatives to index GFR?** 

GFR of 87 ml/min.

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 considered alternative to index the GFR, especially in children.
