Variability of k terms in bioelectrical impedance spectroscopy

Departments of Pediatrics and Clinical Chemistry, Academic Hospital Free University, PO Box 7057, 1007 MB Amsterdam, Netherlands, E-mail: k.demeer{at}azvu.nl

Dear Sir:

Ellis et al described the validation of bioelectrical impedance spectroscopy (BIS) for measurement of volumes of intracellular water (ICW) and extracellular water (ECW) in healthy children (1). When they compared BIS with the dual-energy X-ray absorptiometry (DXA) and total body potassium (TBK) model, new k terms for the BIS computer model in children were derived. Their study is an important contribution to the field, but several issues need to be addressed.

The volume of ECW is arithmetically computed as total body water (TBW) minus ICW (from the DXA and TBK measurements, respectively). Comparison of the mean values for TBW minus ICW and ECW in Table 2 of the article should thus give only differences due to rounding errors. However, for African American males (14–18 y of age), Mexican American males (9–13 and 14–18 y of age), African American females (all age groups), and Mexican American females (9–13 y of age), much larger differences (up to 15 L) are present. Publication of the correct data for all ethnic age and sex groups would be helpful for comparison in future studies. The Bland-Altman comparisons between BIS and the other model are given for combined data of both sexes but not for the age groups. Quantitative data for the age groups in Table 3 would give the reader a sense of the effect of dependency of difference values on the average values. In the BIS results, the mean values for resistance from the Xitron BIS instrument readings would also be helpful for comparison with the authors' previous data (2) and with future results.

The recalculated k terms for the BIS model, as reported by Ellis et al in the present study (eg, for males: k_ECF = 0.330 for the ECW constant and k_P = 3.05 for the ratio of resistivity of intracellular versus extracellular tissues), are different from the values reported by the same group of authors in their previous study (2) in which they compared the Xitron BIS instrument with dilution methods in subjects aged 3–29 y (k_ECF = 0.370 and k_P = 3.03, respectively). Ellis et al did not discuss whether these differences have any practical significance, which k terms for the BIS instrument under study should now be preferred in children, and whether the k terms were different between ethnic groups.

BIS has many advantages in children, including the absence of radiation exposure and the possibility of repeated measurements in the same individual. Elucidation of the issues raised above would be of great value for the users of this promising method in the fields of pediatrics and nutrition.

REFERENCES

1. Ellis KJ, Shypailo RJ, Wong WW. Measurement of body water by multifrequency bioelectrical impedance spectroscopy in a multiethnic pediatric population. Am J Clin Nutr 1999;70:847–53.
2. Ellis KJ, Wong WW. Human hydrometry: comparison of multifrequency bioelectrical impedance with ²H₂O and bromine dilution. J Appl Physiol 1998;85:1056–62.