Novel equations for bioimpedance spectroscopy to calculate body fluid volume based on the agreement between body weight and extracellular water change before and after hemodialysis as a guide

Background MLT-550N is a device to measure the body fluid volume based on bioimpedance spectroscopy. When extracellular water (ECW) or total body water (TBW) is measured with MLT-550N before and after hemodialysis, the change in ECW (ΔECW) or TBW (ΔTBW) is markedly larger than the change in body weight (ΔW). Good agreements between ΔECW and ΔTBW calculated by Moissl equations with ΔW were reported. The aim of this study was to develop novel equations to calculate the body fluid volume by modifying Moissl equations. Methods A total of 466 measurements of 351 hemodialysis patients were used to develop the novel equations. The equations were based on the agreement between ΔECW and ΔW as a guide. The volume of intracellular water was postulated to be constant during hemodialysis. For evaluation of the equations, 5485 measurements of 627 patients were used. Agreements between values of two groups were assessed with Lin’s concordance correlation coefficients. The normal edema ratio (ER = ECW/TBW) corrected with the fat ratio (fcER) according to the parameters of normally hydrated lean and adipose tissues was used as the reference for normal hydration. The fluid status at dry weight was considered to be close to normal. The absolute values of the deviated ratio of actual ER (aER) from fcER (dER = (aER−fcER)/fcER) were compared. In this study, 1 L of body water corresponded to 1.02 kg of body fluid according to a previous report. Results The concordance correlations between ΔW and 1.02∙ΔECW or 1.02∙ΔTBW with the novel equations were higher than with the MLT method or Moissl equations (0.896, 0.596 vs. 0.411, 0.375 or 0.813, 0.411, respectively). The median value of dER with the novel equations was the lowest (0.062) compared with those of the MLT method and Moissl equations (0.164 and 0.144) (p