How should renal hemodynamic data be indexed in obesity?

Clearance data are customarily indexed to body surface area of 1.73 m2. This study examined whether this standard procedure gives correct values for renal perfusion in obese subjects. In 215 subjects who varied in age, gender, height, weight, obesity, and mean arterial blood pressure, RPF was determined by measuring the clearance of (131I)para-aminohippuric acid. Multiple regression analysis of the whole study group revealed that age (beta = -0.44, P < 0.001), height (beta = +0.25, P < 0.01), and arterial blood pressure (beta = -0.19, P < 0.01) were independent predictors of RPF, but that weight or body mass index was not. When related to body surface area, RPF appeared to decline with increasing obesity as follows: normal weight, 609 +/- 153 mL/min per 1.73 m2; overweight, 572 +/- 149 mL/min per 1.73 m2; severely overweight, 530 +/- 145 mL/min per 1.73 m2 (P < 0.012). In contrast, RPF related to height reflected a pattern concordant with the multiple regression analysis: normal weight, 3.76 +/- 0.9 mL/min per meter; overweight, 3.86 +/- 1.0 mL/min per meter; and severely overweight, 3.86 +/- 1.0 mL/min per meter (not significant). A separate repetition of the whole analysis for both normotensive (N = 55) and hypertensive subjects (N = 160) revealed a result similar to that found for the whole group. Thus, our results show that obesity was not a determinant of RPF, and when related to body surface area, inappropriately low values of RPF were calculated for obese patients. It was concluded that RPF values correlate with height and not with surface area in obese subjects.