Comparison of methods to estimate haemodialysis urea clearance

Dialysis adequacy is traditionally measured by monthly blood urea sampling and calculating sessional Kt/Vurea. Modern dialysis machines can estimate clearances each session, so we wished to compare online measurements with standard Kt/Vurea.

Urea clearance was estimated by intermittent changes in effective ionic dialysance and by continuous ultraviolet light absorption spent during the mid-week dialysis session. Total body water was calculated by the Watson equation and measured by multifrequency bioimpedance.

We compared Kt/Vurea measurements in 162 patients with online assessments: 38 by ultraviolet absorption and 124 by effective ionic dialysance (50 Fresenius 4008 and 74 Fresenius 5008). All online measurements overestimated single-pool Kt/Vurea (ultraviolet absorption mean bias 0.25 ± 0.24, effective ionic dialysance 4008H 0.25 ± 0.21 and 5008H 0.20 ± 0.25; p < 0.001). However, there was no difference between dual-pool Kt/V and ultraviolet absorbance (1.28 ± 0.26 vs 1.29 ± 0.27) or by effective ionic dialysance with the 4008 (1.40 ± 0.26 vs 1.46 ± 0.33), although the effective ionic dialysance 5008 overestimated clearance (1.39 ± 0.27 vs 1.31 ± 0.22; p < 0.01). Similarly, with dual-pool Kt/Vurea, the mean bias for ultraviolet absorption was 0.08 ± 0.35, for effective ionic dialysance (EID) 4008 was 0.13 ± 0.55 and for EID 5008 was -0.2 ± 0.36. Hence, the mean bias was greater with the EID 5008 compared to ultraviolet absorption (0.08 ± 0.35 vs -0.2 ± 0.36 vs p < 0.01).

Online measurements allow dialysis adequacy to be measured every session. We found that although online clearances overestimated single-pool Kt/Vurea measurements, there were no significant differences between the continuous ultraviolet light absorbance method and intermittent effective ionic dialysance.