Estimating Total Body Water in Children on the Basis of Height and Weight

Abstract

Abstract. An estimate of total body water (TBW) has important implications in clinical practice. For patients on peritoneal dialysis (PD), the estimate is critical when determining the delivered dialysis dose. The formulas of Mellits and Cheek have been recommended to estimate TBW in children on PD. These formulas were derived from healthy children, and very few infants were included. To assess the accuracy of these formulas, the original data were obtained and additional data from a broad literature review were compiled. The majority of the new data points were in the infant age range. Data were fitted using least-squares methodology and backward elimination to obtain a parsimonious model. Best fits were obtained using age, gender, and weight or a height ϫ weight term. When compared with the previous Mellits and Cheek formulas, the new formula fits better for infants (comparison of prediction errors, P Ͻ 0.0004). These newer formulas do not perform significantly better for the older two groups. Actual TBW measurement in children on PD must still be determined to verify the use of these formulas and to accurately assess dialysis delivery and adequacy.Knowledge of total body water (TBW) has implications for many areas of clinical practice, such as parenteral fluid therapy (1), pharmacokinetic evaluations, and calculation of the delivered dose of dialysis (2). There are a number of methods used to determine TBW directly (3-6); these include the measurement of distribution of "heavy" water (either D 2 O or H 2 0 [18]), bioimpedance analysis (BIA), and estimates from anthropometrically determined fat mass. The accepted criterion standard measurement of TBW is the use of heavy water. Although not difficult to perform and free from radiation risk because the isotopes are stable, the latter studies are time-consuming and costly. BIA has become an accepted method, but there are some important caveats to performing studies in this manner, and some significant variations in the results are inherent when compared with the heavy water (4).The need to accurately estimate TBW in patients on dialysis has taken on great significance during the past few years (7,8). An estimate of TBW is critical for the calculation of Kt/V urea, a measure of delivered dialysis dose that has become the accepted standard. The V component of this term represents the volume of distribution of urea, widely accepted to equal TBW. For patients on hemodialysis (HD), the V component is calculated as a part of the modeling program used to generate the term Kt/V. In contrast, for patients on PD, it is necessary to estimate V. Among children who require dialysis, many of whom receive PD, an accurate estimate of V is, in turn, clearly desirable. The National Kidney Foundation has established a series of quality of care guidelines through the Dialysis Outcomes Quality Initiative (K/DOQI). The K/DOQI guidelines for peritoneal dialysis adequacy recommend that V (TBW) in children should be estimated using the anthropometric formulas of Mellits and Cheek, becaus...

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