Assessing total body and extracellular water from bioelectrical response spectroscopy

We developed and validated assessments for total body water (TBW) and extracellular water (ECW) by using two resistance values of a new electric circuit model (CM) (two resistors; a capacitor and an inductor) with or without body mass. Fluid shifts occurring after 40 min of supine rest did not increase the validity of either estimate. CM estimates were valid; r = 0.941 to 0.969, low SE of estimates of 1.15-2.28 kg, nonsignificant mean differences (CM - dilution; %delta = -0.4 to 1.3%) that were close to the expected measurement errors for TBW (+/- 1%) and ECW (+/- 5%), and Bland-Altman pairwise comparisons that showed equivalence between methods. The CM estimates of TBW and ECW had marginally better validity than the previously published bioimpedance models. The advantage of the CM model is its assessments of multiple fluid spaces and that it does not require gender-specific equations. We conclude that CM estimate of TBW is acceptable, whereas further validation is needed before the ECW estimate should be used in a clinical or research setting.