Urea kinetic modeling in continuous peritoneal dialysis patients. Effect of body composition on the methods for estimating urea volume of distribution.

Body composition can have a significant effect on methods for estimating urea volume (V) and fractional clearance (KT/V) in patients with renal failure. These effects were examined in 27 men on continuous ambulatory peritoneal dialysis (CAPD). Urea volume was calculated as 0.6% of body weight (V.6), by the Watson formula (VW) and by the Hume formula (VH). Patients were classified as obese (> 120% of ideal body weight), normal (90%-120% of ideal weight), or wasted (< 90% of ideal weight). A patient was considered adequately dialyzed if his weekly KT/V urea exceeded 1.70. Only 75% of subjects were classified as adequately or inadequately dialyzed by all three methods. Six subjects had KT/V0.6 < 1.70 and KT/VW > 1.70; five subjects had KT/VH < 1.70 and KT/VW > 1.70; and two subjects had KT/V0.6 < 1.70 and KT/VH > 1.70. In the obese group, KT/V0.6 (1.62 +/- 0.67) was significantly less than KT/VW (1.93 +/- 0.76; p < 0.001) and KT/VH (1.92 +/- 0.78; p < 0.001). Fractional errors in V and KT/V between any two methods were highly correlated with percent deviation from ideal body weight (r = 0.81 or higher). In peritoneal dialysis patients, excessive body weight affects the estimates of V and KT/V urea by certain methods, and may lead to erroneous impressions about the adequacy of CAPD.