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Evaluation of between-methods agreement of extracellular water measurements in adults and children^1,2,3

¹ From the New York Obesity Research Center, St Luke's–Roosevelt Hospital, Columbia University Institute of Human Nutrition, College of Physicians and Surgeons, New York, NY (SBH, DG, JA, XFP-S, RNP, JW, SH, and ZW), and the Exercise and Health Laboratory, Faculty of Human Movement–Technical University of Lisbon, Lisbon, Portugal (AMS and LBS)

Background: Extracellular water (ECW), a relevant molecular level component for clinical assessment, is commonly obtained by 2 methods that rely on assumptions that may not be possible to test at the time the measurements are made.

Objective: The aim of the current study was to evaluate the degree of agreement between ECW assessment by the sodium bromide dilution (ECW_NaBr) and total body potassium (TBK; whole-body ⁴⁰K counting) to total body water (TBW; isotope dilution) methods (ECW_TBK-TBW) in an ethnically mixed group of children and adults.

Design: ECW was measured with the ECW_NaBr and ECW_TBK-TBW methods in 526 white and African American males and females (86 nonobese children, 193 nonobese adults, and 247 obese adults). Fat mass was assessed with dual-energy X-ray absorptiometry. Multiple regression analysis was used to examine the variables related to between-ECW method differences.

Results: Significant but generally small group mean (±SD) differences in ECW were found in the obese adults (1.28 ± 2.54 kg) and children (–0.71 ± 1.78 kg). The magnitude of the differences was related to mean ECW in obese adults, children, and nonobese adults, and the relations between these variables were modified by sex for nonobese adults. ECW differences were also dependent on age, weight, sex, and race or on interactions between these variables.

Conclusions: Overall, although good between-method agreement was found across the 3 groups, the degree of agreement varied according to subject characteristics, particularly at the extremes of ECW and body weight. We advance a possible mechanism that may link subject characteristics with the degree of agreement between ECW measurement methods and their underlying assumptions.