Ionic dialysance and the assessment of Kt/V: the influence of different estimates of V on method agreement

Background. Ionic dialysance was recently introduced as a means to assess Kt/V (K(ID)t/V). With this method, urea distribution volume (V) has to be estimated. The primary aim of the present study was to assess the agreement between equilibrated Kt/V assessed by urea kinetic modelling (eKt/V) with K(ID)t/V taking into account different estimates of V, and to assess the monthly variation in V. Secondly, the mechanisms behind the intra-treatment changes in ionic dialysance and inter-treatment variability of K(ID)t/V were assessed. Methods. Sixty-six patients were included. eKt/V was estimated using 30min post-treatment sampling in the second generation Daugirdas equation. V was assessed by the formulae of Watson and Chertow (V-watson;V-Chertow), double-pool urea kinetic modelling (V-UKM) and by ionic dialysance (V-IOD) [Diascan; Hospal((R))]. Results. The use Of VUKM or VIOD instead Of Vwatson or V-chertow improved the relation between eKt/V and K(ID)t/V (both r=0.93; P < 0.001 vs r=0.84 and r=0.81; P < 0.001). Mean values of eKt/V (1.19 +/- 0.21), K(ID)t/V-UKM (1.19 +/- 0.30) and K(ID)t/V-IOD (1.21 +/- 0.25) were comparable. Intra-class correlation coefficient of VIOD was 0.87 with a 1-month interval and < 0.75 after 2 and 3 months. Intra-class correlation coefficient of V-DP was 0.79 with a 1-month interval and < 0.75 after 2 and 3 months. Inter-treatment variation in K(ID)t/V during six consecutive dialysis sessions was 6.1% +/- 0.6%. Changes in blood flow were the main determinant of variations in K(ID)t/V (P < 0.05). During treatment, ionic dialysance decreased by 12 +/- 13 ml/min (P < 0.001). The decline in blood volume was the major determinant of the intra-dialytic change in ionic dialysance (P < 0.05). Conclusion. The use Of VIOD and V-UKM results in better agreement between eKt/V and K(ID)t/V compared with anthropometric formulae. K(ID)t/V was comparable with eKt/V and thus lower than expected for a single-pool method. V-IOD and V-UKM, should be assessed at least monthly. K(ID)t/V varies widely between consecutive dialysis sessions, mainly due to differences in blood flow. During treatment, ionic dialysance decreases, which is related to the relative decline in blood volume.