Accuracy of Kt/V estimations in high-flux haemodiafiltration using per cent reduction of urea: incorporation of urea rebound

The estimation of Kt/V by utilization of the pre- and postdialysis urea concentrations (per cent reduction in urea and In(Upre/Upost)) provides a simple, quick technique that can be applied at the bedside. However, the accuracy of such techniques has been questioned. One possible reason for this inaccuracy may be the frequently observed postdialysis rebound in serum urea. We assessed the urea rebound at 30 min postdialysis in 34 haemodiafiltered patients and compared the calculation of Kt/V using this urea concentration with that using the immediate postdialysis concentration. These results were then compared to the Kt/V calculated by urea kinetic modelling (UKM), also utilizing the delayed serum urea concentration. The degree of urea rebound observed was large, 21.4%, being a reflection of the short-duration, rapid-flux dialysis. The formulae for calculation of Kt/V all significantly correlated with Kt/V by UKM but all gave results significantly different from Kt/V by UKM (P < 0.001 by paired t test). For assessment of Kt/V by these formulae or by UKM, the urea rebound is too large to ignore in the setting of short-duration, rapid-flux dialysis.