Objective: To investigate the accuracy of total body water (TBW) predicted by foot-to-foot bioelectrical impedance compared with a deuterium oxide dilution technique in oncology outpatients receiving radiotherapy.
Design: Cross-sectional design.
Setting: Two private Australian radiation oncology facilities.
Subjects: In all, 27 subjects (23 males; four females); mean age 62 (+/-15) y; mean BMI 26.2 kg/m2 (+/- 3.6).
Intervention: TBW was measured using a deuterium oxide dilution technique and predicted using foot-to-foot bioelectrical impedance (Tanita Inc., Tokyo, Japan, Models TBF 410 and 300GS).
Results: The mean (s.d.) values for predicted and measured TBW was 41.5 (6.7) and 39.7 (8.7) l, respectively, indicating a mean bias to overestimation by the foot-to-foot impedance of 1.8 l. However, a significant negative correlation between the mean of the measurements of TBW and their difference (r=-0.40; P=0.04) indicates a progressive underestimation of TBW by foot-to-foot impedance as the water content of the body increases. The analysis of 95% limits of agreement (+/-2 s.d.) showed that for most individuals the TBW predicted by foot-to-foot impedance can vary as much as 12 l above or 8.6 l below the actual TBW measured by a deuterium oxide dilution technique.
Conclusion: There is good agreement between foot-to-foot bioelectrical impedance with a gold standard technique at the group level. However, the significant correlation between the difference of predicted and measured TBW, and the wide limits of agreement between the two methods, indicates that the use of foot-to-foot impedance in assessing TBW may lead to unacceptable error in individuals.