Variability of body density and body impedance at different frequencies.

Body density and body impedance at 15 selected frequencies ranging from 1 to 1350 kHz were determined in 13 subjects on four consecutive days of the week. Bioelectrical impedance at the same frequencies was also determined in 11 subjects on the same day of the week during four consecutive weeks. Day-to-day variability of body density and body impedance and week-to-week variability of body impedance were studied. Statistical analyses did not reveal significant day- or week-effects in variabilities. The mean within-person variation in body density between days was 0.0019 kg/l (CV 0.2%). Mean within-person variation in body impedance between days was 57 ohms (CV 8.7%) at 1 kHz. At 5, 50 and 100 kHz the within-person variation between days was 16 ohms (CV 2.4%), 12 ohms (CV 2.1%) and 11 ohms (CV 2.0%), respectively. Mean within-person variation between weeks was 44 ohms (CV 2.4%) at 1, 5, 50 and 100 kHz, respectively. Overall the within-subject variability in impedance at all frequencies was higher in females. The larger mean within-person variation in impedance at 1 kHz could only partly be explained by variation in body weight and may be a real error in the measurement. Electrodes with a larger surface area reduced the variability to values comparable with those at higher frequencies. The day-to-day variation in impedance at higher frequencies causes an error in calculated FFM, which is about two times larger compared to the estimated error in FFM by body density.