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