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/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.