ASYMMETRY OF THE TOTAL-BODY WATER PREDICTION BIAS USING THE IMPEDANCEINDEX

Our purpose was to prove on a geometric basis that the bias of total b ody water (TBW) prediction equations based on the impedance index is f ar greater in fluid overloading than in dehydration. We used formal ev aluation of conventional bioimpedance regression equations in both nor mal and abnormal body fluid status. We plotted the hyperboloid functio n generated from a standard prediction equation for the TBW over the r esistance-reactance (RXc) plane containing the bivariate tolerance int ervals (ellipses) of the healthy population. The equation estimated 35 L TBW for the average man (both sexes) of 170 cm height. Leaving the center of the tolerance ellipses, over which the function was relative ly flat, the predicted TBW rapidly increased to absurd values for the shorter vectors, indicating fluid overloading (e.g., > 100 L for R < 1 70 ohm). Migration of the longer impedance vectors beyond the upper po le of 95% tolerance ellipse, which is in the dehydration region, produ ced less biased estimates of TBW (e.g., < 22 L for the extreme R value s > 850 ohm). Different formulas produced TBW prediction bias of the s ame order. Due to the hyperbolic shape, functions of the impedance ind ex are critically dependent on the region of the RXc plane where they are calculated and they produce misleading results in patients with fl uid overload. (C) Elsevier Science Inc. 1997.