Effects of procedure, upright equilibrium time, sex and BMI on the precision of body fluid measurements using bioelectrical impedance analysis

Background/Objectives: Extensive work has addressed the validity of bioimpedance (BIA) measurements and the effect of posture on fluid homeostasis. However, limited research has investigated effects of subject preparation. This study aimed to determine the precision of total body water (TBW) and extracellular water (ECW) measurements using a stand-on multifrequency BIA (MFBIA seca mBCA 514/515), in three pre-test procedures: supine, sitting, and following walking, with specific reference to the influence of sex and BMI. Subjects/Methods: Fifty three healthy, ambulatory men (n=26, age:32.5±9.4yrs) and women (n=27, age:35.2±10.3yrs) received repeat MFBIA measurements (six measurements from 0 to 15 min). Agreement and precision were evaluated for each condition and time point. Results: Significant TBW sex differences from supine posture were observed for walking (females) and sitting (males) postures. For BMI (≤ 24.9 kg.m-2) significant TBW differences from supine were observed for both sitting and walking and significant ECW differences from sitting were also observed with both supine and walking. There was no significant effect of sex or BMI (≥ 25.0kg.m-2) on ECW measures. Irrespective of sex or BMI, there was close agreement in TBW and ECW precision over the three protocols. Conclusions: Practitioners can have confidence in the precision of TBW and ECW measurements within a 15 minute time period and pre-testing conditions (supine, sitting or walking) in healthy subjects, though must be cautious in assessments when pre-test postures change. Further research to examine the impact of pre-testing procedures on stand-on MFBIA BIA measurements, including subjects with fluid disturbance, is warranted.