Normalization of clearances in peritoneal dialysis using a formula for body water derived from an end-stage renal disease population

Abstract

Objective: To compare body water (V) estimates from the Chertow formula (V(C)), which was derived in an end-stage renal disease population, to V estimates from the Watson formulas (V(W)) in continuous ambulatory peritoneal dialysis (CAPD) patients. To identify CAPD patients in whom V(C) is preferred to V(W) for clearance studies. Design: Retrospective analysis of clearance studies. Setting: Dialysis units of four academic medical centers. Participants: 302 subjects on CAPD. Intervention: 613 clearance studies by standard methods. Main Outcome Measures: Comparisons between V(C) and V(W)' and between urea clearance normalized by V(C) [(Kt/V(C))(ur)] and V(W) [(Kt/V (W))(ur)]. Results: V(C) exceeded V(W) by 3.5 ± 1.6 L (p<0.001), or 9.6% on average. This degree of overestimation of V(W) is in the range of body water estimates found in CAPD subjects with severe volume overload (> 5% of body weight) in previous studies. Total (Kt/W(V))(ur) exceeded total (Kt/V(C))(ur) by 8.6%. By linear regression, V(C) = -0.589 + (1.112 x V(W)), r = 0.983. VW exceeded V(C) in only 12 studies. Young age, short height, low body weight, and low prevalence of diabetes characterized the studies with V(W) > V(C). Total (Kt/V(W))(ur) was adequate (≥2.0 weekly) in 276 studies. Among these, 74 studies had inadequate total (Kt/V(C))(ur) (<2.0 weekly). By logistic regression, the predictors of inadequate (Kt/V(C))(ur), when (Kt/V(W))(ur) was adequate, included the presence of diabetes, great height, and long duration of CAPD. Conclusions: V(C) provides estimates of body water exceeding those provided by V(W) in a great majority of CAPD patients. Consequently, approximately 25% of the clearance studies that are adequate when V(W) is used as the normalizing parameter may be inadequate when V(C) is used. V(C) may provide a more appropriate estimate of body water than V(W) in CAPD patients with volume overload.