Body composition studies in intensive care patients: comparison of methods of measuring total body water.

Changes in total body water (TBW) were monitored in 12 critically-ill intensive care patients using four independent methods. Over the 10-day study period TBW measured by tritium dilution changed from 51.3 ± 2.5 (SEM) kg to 43.6 ± 2.3 kg, an average loss of 7.7 ± 0.8 kg. A six-compartment model of the body incorporating measurements of protein by in vivo neutron activation analysis and fat and bone mineral by dual-energy X-ray absorptiometry was used to determine TBW by difference from body weight. The 10-day change in TBW measured by this approach was 8.4 ± 0.9 kg which correlated well with the tritium dilution changes (r=0.84, P<0.01, SEE=1.83 kg). The changes measured by single frequency and multi-frequency bio- electrical impedance analysis were not significantly different from the tritium results (9.7 ± 1.3 and 8.2 ± 0.8 kg. respectively) although the prediction errors were high for both methods (SEE=3.29 and 2.72 kg, respectively) with correlations that were statistically significant for the single frequency approach but not for the multi-frequency approach (r=0.71, P<0.01 and r=0.45, ns, respectively). The high prediction errors render these impedance techniques inappropriate, at the present time, for monitoring total water changes in individual intensive care patients.