Influence of various factors on the measurement of multifrequency bioimpedance.

Body impedance values at various frequencies (from 1 to 100 kHz) were determined in 104 subjects on seven separate days over a two-week period. The variability of body impedance in different measurement conditions was studied. In particular, the effects of the electrode locations, the ingestion of some substances (sugar, alcohol, mineral salts), body spatial geometry, the time spent in the supine position and the menstrual cycle were assessed. Under standardized conditions (in the morning, in the fasting state, with an empty bladder and with the body in a standardised spatial position), the within-subject day-to-day variability was 3-14 Ohms. Under different experimental conditions, the within-subject variability was generally much higher. This was particularly evident for female subjects. We observed significant mean variations in relation to the different experimental factors introduced one at a time, with the exception of the menstrual cycle. For example, half an hour after the intake of various substances, body impedance had generally increased by 6-17 Ohms in comparison with values in the fasting state. Changes in body impedance during the menstrual cycle, however, were small and never statistically significant. The impedance variations obviously caused significant changes in estimated parameters of body composition. It is concluded that controlled conditions and standardization of multifrequency bioimpedance analysis (MBIA) methods are indispensable for the application of this technique.