Dependence of peritoneal clearances on body size in continuous ambulatory peritoneal dialysis: Effect of the normalizing size indicator

In peritoneal dialysis (PD), small solute clearances are normalized by body water (V) and body surface area (BSA). The purpose of this study was to id entify a V or BSA produced stronger associations between body size and norm alized clearances. We studied the relationship between four size indicators (V, BSA, height, and weight) and either peritoneal urea clearance normaliz ed to V (Kt/ V-ur) and BSA (C-ur) or creatinine clearance normalized to V ( Kt/V-cr) and BSA (C-cr). A total of 613 clearance studies were performed in subjects on continuous ambulatory peritoneal dialysis (CAPD) with four dai ly exchanges and a 2 L fill volume. As size increased, the normalized perit oneal clearances decreased in a nonlinear fashion (regression: y = b(0) + b (1)x(-1), where x is a size indicator and y is a normalized clearance). Sig nificant (p < 0.001) negative correlations were found between each normaliz ed clearance and each size indicator. However, in each case, the correlatio n was higher when V, rather than BSA, was used. For example, BSA correlated more closely with Kt/V-ur(-0.660) than C-ur(-0.556), and also with Kt/V-cr (-0.579) than C-cr(-0.446). Normalized clearances are smaller in targe subj ects on CAPD because one mathematic determinant of the clearance, the drain volume (Dv) normalized by V (Dv/V) or BSA (DV/BSA), decreases as size incr eases. The relationship between Dv/V or Dv/BSA and the size indicators was studied by the same nonlinear regression model. The correlations of the siz e indicators with Dv/V were also consistently higher than the corresponding correlations with Dv/BSA In subjects who were on the same PD schedule, the dependence of clearances on size was consistently higher when V, rather th an BSA, was the normalizing parameter. Because prescription of the dose of PD is based on body size, there is a practical advantage by using V as the sole normalizing parameter for both urea and creatinine clearance.