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Real-time classification of hydration in patients with altered fluid status remains a priority with increasing efforts focused on bioelectrical impedance vector analysis (BIVA) because of the reliance on impedance measurements independent of body weight and sample-specific regression equations to predict fluid volumes.12 Oh et al.3 derive reference BIVA plots for healthy Korean adults and identify population-specific differences in impedance vector positions and distributions on resistance-reactance (RXc) plots. They attribute these disparities principally to ethnic differences in body composition and health status and only mention in passing the potential impact of impedance instruments as a causal factor.
Variations in the technical characteristics of the bioimpedance analyzers used to obtain the BIVA reference values (Table 2 and Fig. 3) include impedance technology (single frequency, phase-sensitive compared to multiple frequency spectroscopy with modeling), electrode placement (whole-body or wrist-to-ankle compared to segmental electrode location), electrode type (hydrogel versus metal), and calibration (point or single frequency compared to modeling of a wide spectrum of frequencies) that directly affect in vivo impedance measurements. Investigators report significant and biologically meaningful discrepancies exceeding 10% in R and up to 20% in Xc with various impedance instruments compared to the 50 kHz phase-sensitive instrument used in development of the BIVA.456 Additionally, failure to use contact electrodes with low inherent impedance results in mis-classification of hydration with BIVA.7
The observed disparity in BIVA reference data on the RXc plot (Fig. 3) highlight apparent population differences in body composition confounded with a moderating effect of impedance device. Data for the Italian and Spanish samples came from a 50 kHz phase-sensitive device.89 In contrast, a phase-sensitive 50 kHz impedance unit with questionable calibration10 provided measurements to construct the US population reference BIVA data11 while a multi-frequency spectroscopy device yielded modeled impedance data for the Korean sample.3
Whereas individual impedance device measurement reliability is generally high, an absence of international manufacturing standards, coordination of technology and cross-calibration of electrical accuracy contributes to significant inter-instrument measurement variability.1213 These findings emphasize the lack of inter-changeability among bioimpedance instruments and underscore the need for consistency in the selection of a bioimpedance instrument in research and clinical applications of hydration assessment in which standards for classification and evaluation of the effects of treatment are needed.
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