Background: One of the uncertainties in the prescription of dialysis dose, measured as Kt/V for urea (Kt/Vu), is the volume of urea distribution (V). The aim of our study was to compare two different approaches to estimating V and Kt/V, respectively, and to compare the predicted (prescribed) dose with the actually delivered dose, measured by urea kinetics.
Material/methods: V was estimated using anthropometric measures with the Watson formula (VWatson), and also by multi-frequency bioimpedance analysis (VBIS). Both volumes were determined for the state at the end of dialysis and ultrafiltration, and then used to predict (prescribe) Kt/VWatson and Kt/VBIS, respectively. The delivered Kt/Vu was determined from pre- and postdialysis urea concentrations.
Results: 209 patients were studied in 254 measurements. VWatson and VBIS correlated significantly (VBIS=0.64*VWatson+15.03, r2=0.67) but VWatson (36.0 +/- 6.4 L) was higher than VBIS (32.5 +/- 8.1 L). The bias between techniques was 3.5 +/- 4.6 L. As a consequence of volume underestimation, Kt/VBIS was overestimated (1.44 +/- 0.38) when compared to the actually delivered Kt/Vu (1.28 +/- 0.32). However, Kt/VWatson (1.27 +/- 0.25) based on VWatson was almost identical to the actually delivered treatment dose.
Conclusions: The close correspondence between the delivered and predicted (prescribed) dose of dialysis using anthropometric volumes leads to the conclusion that in this study kinetic distribution volume was best described by VWatson. The overestimation of prescribed dialysis dose based on bioimpedance analysis suggests caution in the use of bioimpedance volumes, because of the risk of prescribing inadequate dialysis.