Gender differences in normalized clearances in CAPD: Role of body size andnormalizing parameters

Objective:To compare raw (not normalized) and normalized urea and creatinin e clearances between women and men on continuous ambulatory peritoneal dial ysis (CAPD). To study whether potential gender differences are due to the n ormalization 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 CAP D with four daily exchanges and a 2-L exchange volume. Intervention: Measur ement of urea and creatinine clearances (261 in women, 352 in men) by stand ard methods. Body water (the volume of distribution, V, for both urea and c reatinine) 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/V-ur), while creatini ne clearances was normalized by both V (Kt/V-cr) and body surface area (BSA ) (C-cr). 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; C-cr: women 61.7 L, men 78.3 L). Raw renal clearances were higher in men, while raw per itoneal clearances were comparable. Mean weekly total KtN(ur) was higher in women (2.19 vs 1.94 in men), mean weekly total Kt/V-cr did not differ betw een the genders (women 2.01, men 1.95), while mean weekly C-cr was higher i n men (73.0 vs 64.7 L/1.73 m(2) in women). When clearances differed, the di fferences 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 cle arance values in women, while normalization by BSA creates relatively highe r clearance values in men. Thus the normalization process may create artifi cial differences in the normalized clearances between genders.