Higher body mass index is associated with higher fractional creatinine excretion in healthy subjects

Background. Accurate glomerular filtration rate (GFR) measurement in normal to high range is important for epidemiological studies and workup for kidney donation. Creatinine-based equations perform poorly in this GFR range. Creatinine clearance (CrCl) provides a substitute, provided urine is collected accurately and tubular creatinine handling can be accounted for. The latter is poorly characterized in the normal GFR range.

Methods. Therefore, we studied performance of CrCl, fractional creatinine excretion (FEcreat) and its determinants in 226 potential kidney donors (47% males, mean 53 +/- 10 years). GFR was assessed as I-125-iothalamate clearance, simultaneously with 2-h CrCl and 24-h CrCl.

Results. Mean GFR was 101 +/- 18, 2-h CrCl 110 +/- 20 and 24-h CrCl 106 +/- 29 mL/min/1.73 m(2). Mean bias of 24 h CrCl was 7.4 [inter-quartile range -6.7 to 20.0] mL/min/1.73 m(2), precision (R-2) 0.39 and 30% accuracy 82%. Mean FEcreat was 110 +/- 11%. FEcreat correlated with body mass index (BMI) (r = 0.34, P <0.001). Consequently, bias of 24-h CrCl increased from 2.7 (inter-quartile range -6.5 to 16.7) to 8.6 (inter-quartile range -5.8 to 20.5) and 12.6 (inter-quartile range 7.0 to 25.4) mL/min in subjects with BMI <25, 2530 and >30 kg/m(2), respectively (P <0.05). On multivariate analysis, BMI and gender were predictors of FEcreat.

Conclusions. CrCl systematically overestimates GFR in healthy subjects. The overestimation significantly correlates with BMI, with higher FEcreat in subjects with higher BMI. The impact of BMI on tubular creatinine secretion can be accounted for, when using CrCl for GFR assessment in the normal to high range, by the following formula: GFR = 24-h CrCl - (22.75 + 0.76 x BMI - 0.29 x mean arterial pressure (-6.11 if female).