Total body water measurement by a modification of the bioimpedance spectroscopy method

We propose a method for calculating directly total body water (TBW) volumes (V-t) from whole body resistance extrapolated at infinite frequency (R-infinity) using a XITRON 4200 impedance meter. Mean TBW resistivities for men and women were determined from measurements of R-infinity and fat-free mass (FFMd) measured by DXA in 58 healthy subjects assuming an average hydration coefficient of 73.2%. Mean differences between V-t measured by our new method and those deduced from DXA data were +0.11 +/- 1.61 L for women and +0.13 +/- 2.16 L for men. For validation, this method was tested with the same resistivities against a 2nd group of 16 volunteers and the mean difference between V-t from impedance and DXA was -0.80 +/- 1.43 L. Since the resistance at 50 kHz (R-50) was found to be equal, in average, to 1.230 R-infinity for men and 1.223 R-infinity for women, this method can also be applied at 50 kHz with a similar accuracy by estimating R-infinity from R-50. When our new method was applied to the monitoring of water loss during 28 dialysis runs performed on 13 patients, it predicted a mean water loss equal to 94% of ultrafiltered volume.