Impedance and reactance — how the measurement actually works

• Directly measured: resistance and reactance for five segments at eight frequencies, and phase angle at three frequencies. Together with weight on the built-in scale, these are the only genuinely measured quantities of the whole examination.
• Calculated: the R/Ht and Xc/Ht ratios. The measured value in ohms is divided by height in metres, giving Ω/m. Height is not measured but entered — if it is entered wrongly, the ratio is wrong too.
• Modelled: R0 and Z∞ are limiting points the device does not transmit. It actually measures from 1 kHz to 3 MHz and calculates the values for zero and infinite frequency from the measured spectrum. The manufacturer documentation does not state exactly how.
• Derived from impedance: total body water, intracellular and extracellular water and their segmental values. The manufacturer presents these as measured quantities rather than estimates.
• The manufacturer documentation does not publish the regression equations that derive water, protein, muscle mass and fat from impedance. It describes the principle but not the formulas — so the values cannot be independently recalculated.
• Age, sex and ethnicity do not enter the body composition calculation. The catalogue states this explicitly: they are used only for reference ranges and scores by which the result is evaluated.

• Neither a higher nor a lower value is generally better. An isolated figure establishes neither hydration nor any health state.
• Resistance and reactance differ by segment, entirely as expected. Arms and legs are long and narrow and therefore have high impedance; the trunk is short and wide and has low impedance. Different numbers between arms and trunk are not an error.
• Impedance falls as frequency rises. Comparing a value at 5 kHz with one at 250 kHz therefore makes no sense — the same frequencies are always compared with each other.
• R/Ht and Xc/Ht are meaningful mainly as a pair in a BIVA plot, which accounts for age, sex and reference population. The numbers alone on the result sheet do not show a position in the vector plot.
• Treat these values as the basis the other results arose from rather than a standalone output to be evaluated. Phase angle, body water and the ECW Ratio are far more readable.

• A change only means something between measurements taken under comparable conditions. The NIH consensus statement on bioimpedance treats standardised conditions as a precondition for comparing repeated results.
• A fall in resistance usually corresponds to more body water, a rise to less. It does not say whether the water was added inside or outside the cells — the ECW Ratio serves that purpose.
• Reactance changes more slowly than resistance. Short-term swings therefore usually say more about water balance than about change at the cellular level.
• The Comparison Result Sheet shows a Cole-Cole plot with resistance on the X axis and reactance on the Y axis. The manual states that a shift of the curve to the left corresponds to an increase in body water, and a shift upwards is read as a healthier cell membrane. A standard median curve is displayed alongside the current and previous curves.
• Do not interpret a one-off difference. Electrical values respond to hydration, temperature and the quality of electrode contact faster than body composition does.

General principles: how to read change between measurements →

• Phase angle — the direct output of the relationship between resistance and reactance and the most readable value of this pair.
• Segmental phase angle — the same quantities split across five body parts.
• Total body water and intracellular and extracellular water — the components that arise from impedance first.
• The ECW Ratio — shows how water is distributed between the space inside and outside cells.
• Segmental lean mass — a value whose accuracy depends directly on the quality of the impedance measurement in that segment.

• The device does not scan or image the body. No image is produced; only electrical quantities are measured.
• Fat is not measured. It is estimated as the difference between weight and the fat free mass derived from body water.
• Higher resistance does not mean a worse result. It mainly reflects the length and cross-section of a segment and the amount of water in it.
• Impedance from different devices is not interchangeable. The number of frequencies, the electrode arrangement and the type of contact all differ, so neither the values nor the derived results can be compared.
• Home scales with electrodes only under the feet measure mainly the lower half of the body and calculate the rest. Eight touch electrodes measure arms, trunk and legs separately.
• Zero and infinite frequency are not frequencies the device actually transmits. They are modelled limiting points calculated from the measured spectrum.

• The quality of electrode contact — dry skin or dead cells worsen conductivity. The manual therefore recommends wiping hands and feet.
• Body position and time spent standing. The manual recommends standing for at least 5 minutes before the measurement, because fluid distribution changes with position.
• Stance during the measurement: arms straight and not touching the trunk, thighs not touching each other.
• Room temperature. The manual gives a range of 20 to 25 °C, because circulation changes in both cold and heat.
• Hydration status, food and drink shortly before the measurement, a full bladder.
• Training, showering and sauna shortly before the measurement, which change perfusion and fluid distribution.
• Menstruation, during which body water fluctuates.
• Jewellery, watches and other metal objects; the measurement is taken barefoot.

• You do not need to follow these values. They are input data — for practical interpretation reach for phase angle, body water and the ECW Ratio.
• When comparability of measurements matters to you, keep the same routine: similar time of day, no training or sauna beforehand, after using the toilet, in the same kind of clothing.
• If you compare results from different devices and they do not agree, that is not necessarily an error. Different devices use different frequencies and equations and their values are not interchangeable.
• When you want to know why a result changed, start with the measurement conditions. Electrical quantities respond to them faster than actual body composition does.

• It does not establish health status, hydration or any diagnosis. A resistance or reactance value alone has no clinical reading.
• The equations deriving body composition from impedance are not published, so results cannot be independently verified.
• The manufacturer documentation does not state how the modelled R0 and Z∞ values are calculated and publishes no reference ranges for them.
• Values are not comparable between devices from different manufacturers or between different frequencies of the same measurement.
• The body composition estimate rests on an assumption about the usual water content of lean tissue; where water balance is altered, it shifts.
• Bioimpedance is not a reference method for clinically establishing the amount of fat or muscle.