Background: Despite its importance, total body water (TBW) is usually estimated rather than measured due to the complexity of isotope dilution methods. The aim of this study was to demonstrate the applicability in haemodialysis (HD) patients of a recently developed on-line breath test, previously validated in healthy subjects, that uses the gold standard deuterium dilution method to measure TBW. In particular we wished to show that a pre-dialysis estimation was as good as a post-dialysis equilibrated measurement in order to avoid patients needing to remain behind after dialysis treatment.
Methods: The dispersal kinetics of breath HDO, measured using a flowing afterglow mass spectrometer (FA-MS) following ingestion of D(2)O immediately post-dialysis, were determined in 12 haemodialysis patients and used to calculate the absolute TBW(PostHD) after full equilibration. TBW(PreHD) was then determined from breath samples taken immediately prior to the next dialysis. This measurement was adjusted for the interdialytic weight change and urine output (TBW(PreHD-adjusted)) and compared to the TBW(PostHD). The accuracy and precision of FA-MS was also assessed using known concentrations of deuterium-enriched water samples.
Results: Mean TBW(PostHD) was 50.0 +/- 9.3 L and TBW(PreHD-adjusted) was 50.7 +/- 9.0 L. They were highly correlated (R = 0.99, P < 0.001) with a CV of 2.6%. The mean difference was +0.74 L (SEM 0.35, 95% CI -0.03 to 1.51 L, P = 0.059), compatible with a daily insensible loss of 0.37 L. Accuracy and precision of FA-MS were comparable to the previous validation work.
Conclusions: This non-invasive adaptation of the D isotope dilution method for determining TBW can be applied to haemodialysis patients who show deuterium equilibration kinetics identical to normal subjects; a pre-dialysis estimation may be used to determine TBW, and so avoiding the necessity to remain behind after dialysis making this suitable for application in the clinical setting.