Consequences of Normalizing Peritoneal Dialysis Dose

Gender Differences in Normalized Clearances in Capd: Role of Body Size and Normalizing Parameters

Objective

To compare raw (not normalized) and normalized urea and creatinine clearances between women and men on continuous ambulatory peritoneal dialysis (CAPD). To study whether potential gender differences are due to the normalization process.

Design

Retrospective analysis of clearance studies.

Setting

Dialysis units of four academic medical centers.

Participants

The study included 302 subjects (135 women and 167 men) on CAPD with four daily exchanges and a 2-L exchange volume.

Intervention

Measurement of urea and creatinine clearances (261 in women, 352 in men) by standard methods. Body water (the volume of distribution, V, for both urea and creatinine) was estimated by the Watson anthropometric formulas.

Main Outcome Measures

Comparison of raw and normalized clearances between women and men. Urea clearance was normalized by V (Kt/Vur), while creatinine clearances was normalized by both V (Kt/Vcr) and body surface area (BSA) (Ccr).

Results

Mean values of weekly total (peritoneal plus renal) raw clearances were higher in men (urea clearance: women 67.1 L, men 77.4 L; Ccr: women 61.7 L, men 78.3 L). Raw renal clearances were higher in men, while raw peritoneal clearances were comparable. Mean weekly total Kt/Vur was higher in women (2.19 vs 1.94 in men), mean weekly total Kt/Vcr did not differ between the genders (women 2.01, men 1.95), while mean weekly Ccr was higher in men (73.0 vs 64.7 L/1.73 m2 in women). When clearances differed, the differences were significant at p < 0.001. Men had greater height and weight, while women had greater body mass index. On the average, V in men exceeded V in women by 31%, while BSA in men exceeded BSA in women by only 12%.

Conclusions

Normalization of clearances by V creates relatively higher clearance values in women, while normalization by BSA creates relatively higher clearance values in men. Thus the normalization process may create artificial differences in the normalized clearances between genders.

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 identify if V or BSA produced stronger associations between body size and normalized clearances. We studied the relationship between four size indicators (V, BSA, height, and weight) and either peritoneal urea clearance normalized 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 daily exchanges and a 2 L fill volume. As size increased, the normalized peritoneal clearances decreased in a nonlinear fashion (regression: y = b0 + b1x(-1), where x is a size indicator and y is a normalized clearance). Significant (p < 0.001) negative correlations were found between each normalized clearance and each size indicator. However, in each case, the correlation was higher when V, rather than BSA, was used. For example, BSA correlated more closely with K/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 large subjects 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 increases. The relationship between Dv/V or Dv/BSA and the size indicators was studied by the same nonlinear regression model. The correlations of the size 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 than 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.