Determination of total body water in uraemic patients by bioelectrical impedance

The measurement of total body water by bioelectrical impedance in a group of renal patients was evaluated against the tritium dilution method. The effect of haemodialysis and the presence of peritoneal dialysate on the impedance were also investigated. The correlation between the two methods is r = 0.90 with a residual standard deviation of 3.7. The standard deviation of the differences between the two methods against the means was 3.66 which means that total body water (TBW) estimated by the bioelectrical impedance (BEI) method may be 6.18 l (X +/- 2 SD) above or 8.38 l below the 3H2O method. The BEI method overestimated the actual weight loss after haemodialysis (3.87 +/- 1.7 l versus 2.43 +/- 1.8 l) but underestimated the volume of peritoneal dialysate in situ. The BEI method would not be appropriate for use in assessing total body water and monitoring acute volume changes in patients with renal failure who are on strict fluid restriction.