Gender, degree of obesity, and discrepancy between urea and creatinine clearance in peritoneal dialysis

The effect of gender and degree of obesity on the size indicators V, used to normalize urea clearance (Kt/Vur), and body surface area (BSA), used to normalize creatinine clearance (Ccr), in peritoneal dialysis was studied by: (1) mathematical comparison of the formulae used to estimate V (Watson and Hume) with the Dubois formula used to estimate BSA in peritoneal dialysis; and (2) comparison of percent deviation of BSA (delta BSA%) and V (delta V%) from ideal weight estimates in 933 clearance studies performed in actual patients (555 in men and 378 in women on continuous ambulatory peritoneal dialysis). V was estimated by the Watson formulae and BSA by the Dubois formula in these studies. delta BSA% and delta V% were stratified in 10% increments in deviation of body weight from ideal (delta W%) in these studies. Mathematically, the relationship between V and BSA is not linear. In the same subject, as obesity develops (delta W% increases) and BSA increases in a linear manner, V increases exponentially. In addition, there are substantial differences in the relationship between V and BSA caused by gender. For the same height and BSA, men have a larger V than women. In the clearance studies performed in actual continuous ambulatory peritoneal dialysis patients, the difference between delta V% and delta BSA% increased significantly (P < 0.0001) from the wasted to the obese subjects by one-way ANOVA in both men and women. Normalization of urea and creatinine clearances by different size indicators creates two types of mathematical distortion in the relationship between the two clearances. One distortion is caused by the degree of obesity. The second distortion is caused by gender. Use of the same size indicator to normalize both urea and creatinine clearances would eliminate these distortions.