Normalization of Clearances in Peritoneal Dialysis Using a Formula for Body Water Derived from an End-Stage Renal Disease Population

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/V W ) 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. V W 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.