Applied Nutritional InvestigationsBioelectrical impedance methods in clinical research: a follow-up to the NIH technology assessment conference☆
Introduction
Bioelectrical impedance analysis (BIA) measures the opposition of body cells and tissues to the flow of a radiofrequency alternating electric current. The voltage drop between electrodes provides a measure of impedance, which is low in lean tissue and high in fat tissue and bone. Impedance is the vector sum of the resistance or conductive characteristics of body tissues and the reactance or additional opposition due to the capacitance of cell membranes, tissue interfaces, and non-ionic substances. Many studies have applied the basic technique of BIA to estimate body composition in various clinical and research settings.
In December 1994, the National Institutes of Health (NIH) convened a technology assessment conference to evaluate the validity and interpretation of data derived by single frequency BIA for the estimation of body composition, noting the need for a consensus among experts on the appropriate conditions of use and appropriate applications of BIA.1 A summary of the NIH findings for the single frequency BIA technology were published2, 3 and a complete text of the draft conference statement is available on the Internet as well.4
In October 1997, Serono Symposia USA, Inc., organized a 1-d conference (Bioelectrical Impedance Methods in Clinical Research) and invited eight research scientists and clinicians, with extensive BIA experience, to examine the progress made on single frequency BIA issues raised 3 y earlier at the NIH conference. In part, this meeting met the NIH’s suggestion for periodic reviews of BIA technology and its use. In addition, the participants reviewed recent technologic advances in the field and how these have and will continue to affect the use of BIA in clinical research. A summary of these findings, presented in response to the questions raised at the NIH conference, are presented in Table I (see refs. 5, 6, 7). In addition, new issues related to BIA measurements, both for the individual and for population studies, that have arisen since 1994 are included in Table I (see refs. 5, 6, 7).
Section snippets
Current methods and models
In 1994, BIA measurements at a single-frequency (typically, 50 kHz) were the ‘industry standard’ and this approach continues to be the most frequently used, despite a number of shortcomings noted over the last 3 y. At the NIH technology conference, it was concluded that resistance, adjusted for height, measured at a single-frequency, was related to total body water (TBW) volume. Unfortunately, a number of biophysic assumptions are needed to translate the resistance value to TBW. Among these
Role in clinical research
The 1994 NIH conference focused considerable attention on the most common use of BIA at the time, i.e., mainly the (indirect) estimation of body fat. Although far easier to perform and less expensive than, for example, hydrodensitometry, single-frequency BIA was and should continue to be viewed with caution as a surrogate technique to estimate body fat.34, 35 The bioimpedance method is related to the conducting volume (or mass) in the body. Body fat per se, because it is nonconductive, is not
Establishing population reference norms
The vast majority of BIA validation studies are in healthy populations and have used series BIA techniques, and have often included additional anthropometric parameters besides height to establish the prediction equations. It has been shown that sex- and age-specific equations are needed, particularly for younger populations (age <18 y) and older adults (age >65 y). Recently, multifrequency BIA data obtained from 500 children aged 3–18 y indicated that the assumed constants used to calculate
Establishing guidelines for clinical use
For BIA to extend beyond research applications in the healthy population, standardization of the procedures is essential. The optimal method, though not always practical (in a clinical setting), for obtaining impedance measurements involves having the subject (preferably fasted, but not dehydrated) lie supine for at least 10 min. Despite this requirement, several low-cost instruments, apparently targeted for the general public, have been marketed that require the subject be in a standing
Conclusions
In 1994, the NIH panel “recommend(ed) that a committee of appropriate scientific experts and instrument manufacturers be formed with the goal of developing instrument standards and procedural methods.” Unfortunately, in the 4 y since that conference reached this conclusion, no such panel (officially or otherwise) has been formed, hence no standard methodology exists among the many instruments that are used. The present panel was convened to start to address these issues. Given the range of
Acknowledgements
We are grateful to J. Gertner of Serono Laboratories, Inc. for his scientific interest in this topic, to D. Pherson for organizing the meeting, and to Serono Symposia USA, Inc. for their financial support. We are especially appreciative of the scientific editorial services provided by M. Kienholz in the preparation of this report.
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