Indirect measures of total body water may confound precise assessment of peritoneal dialysis adequacy

Urea kinetic modeling (UKM) has yet to be optimized as a practical tool for assessing adequacy of therapy in continuous ambulatory peritoneal dialysis (CAPD) patients. Watson equation (WV) and 58% body weight (58%WT) estimates of total body water (TBW) are indirect measures likely to yield imprecise estimates of Kt/V. Bioelectrical impedance (BEI) measures body composition as a function of electrical conductance, minimizing fat contribution to TBW. TBW values were highest when measured as 58% WT and lowest when calculated from WV. These differences were most striking in patients with overweight body habitus. BEI-derived TBW correlated best with UKM values. The relationship between BEI-derived and anthropometrically derived TBW was best in patients of normal habitus. Kt/V values were highest when calculated from WV-derived volumes and significantly differed from Kt/V values calculated from BEI-derived and 58%WT volumes. When segregated by habitus, however, only in overweight patients was this pattern of clinical significance. Serial body weight, lean body mass, and TBW remained stable in patients of normal habitus. Overweight patients increased weight by 5%, lean mass by 2%, and TBW by 5%, 3%, and 2% when measured by 58%WT, WV, and BEI respectively. BEI measures of TBW exclude fat mass and thus strengthen the use of Kt/V for assessing dialysis adequacy in CAPD patients of all body weights.