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Comparative measurements of total body water in healthy volunteers by online breath deuterium measurement and other near-subject methods^1,2,3

David Smith, Barbara Engel, Ann M Diskin, Patrik panl and Simon J Davies

¹ From the Centre for Science and Technology in Medicine, School of Postgraduate Medicine, Keele University, Stoke-on-Trent, United Kingdom (DS, AMD, and SJD); the Department of Nephrology, North Staffordshire Hospitals Trust, Stoke-on-Trent, United Kingdom (BE and SJD); and the V ermák Laboratory, J Heyrovsk Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Prague, Czech Republic (PS).

Background: We developed a new near-subject approach, using flowing afterglow–mass spectrometry (FA-MS) and deuterium dilution, which enables the immediate measurement of total body water (TBW) from single exhalations.

Objectives: The objectives were to show the efficacy of the new FA-MS method in measuring TBW in healthy subjects and to compare these measurements with values derived from multifrequency bioelectrical impedance analysis, skinfold-thickness (SFT) measurements, and both recent and historical published regression equations.

Design: After baseline measurement of breath deuterium abundance, 24 healthy subjects ingested 0.3 g D₂O/kg body wt. A second breath sample was taken after 3 h to measure the increase in deuterium, from which TBW was calculated. Bioelectrical impedance analysis was carried out with a multifrequency analyzer, and SFT was measured by a single trained observer. Methods were compared with the use of Pearson’s correlation coefficient and Bland-Altman analyses.

Results: TBW measures obtained by all methods were highly correlated (r = 0.95–0.98, P < 0.001), especially those between FA-MS, SFT measurement, and recent regression equations. The mean values obtained were within 2% of those published for age-matched control subjects and varied by 1–6% when all methods were compared. Systematic bias was greatest when FA-MS was compared with bioelectrical impedance analysis, which tended to underestimate TBW in smaller, female subjects. No bias related to subject size was observed in a comparison of FA-MS with SFT measurement or with more recent regression equations.

Conclusions: FA-MS is a simple and effective new approach to TBW measurement in healthy subjects. The difficulty of using population-derived equations to estimate TBW in individual subjects is emphasized.

Key Words: Total body water • deuterium isotope dilution • flowing afterglow • mass spectrometry • bioelectrical impedance analysis • breath test, noninvasive measurement

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