[Estimation of total body water from routine urea kinetics]

The comparison of total body water obtained by Watson formula with results of bioelectrical impedance (BIA) shows a fair approximation (Delta% = 1.0 +/- 8.45). However, when compared to other literature formulas the BIA values are higher (8 divided by 23%), and this discrepancy needs further analysis. Urea kinetics was computed using as input data for distribution volume both the Watson anthropometrical and the BIA TBW values: the clearance results do not differ significantly (Delta% = 0.74 +/- 8.35). The Kt/V index results are nearly identical (Delta% = 0.12 +/- 1.17), due to a mathematical loop: a wrong input of volume induces a comparable error of the clearance result, while their ratio is unchanged. For practical routine to monitoring dialysis adequacy the Watson anthropometrical formula has proven as a satisfactory simple approach. Periodical BIA determination provides useful informations mainly when the anthropometrical estimation are doubtful and the comparison of data may increase the kinetic reliability.