Determination of urea distribution volume for Kt/V assessed by conductivity monitoring

Kt/V can be calculated continuously during dialysis without blood samples using the ionic dialysance method. Unlike the usual method using blood samples, a precise value for the patients' urea distribution volume is required. This study compared different methods for the determination of urea distribution volume (V) to evaluate their use in Kt/V measurement, based on conductivity monitoring.

Ten patients were studied during 40 dialysis sessions. Total body water and V were determined using bioimpedance spectroscopy (BIS), anthropometric data, and blood-based kinetic data. Ionic dialysance was measured by conductivity monitoring.

Total body water measured by bioimpedance was determined as VBIS= 37.0 +/- 7.1 L or 49.6 +/- 4.4% of body weight. V determined using ionic dialysance as input to urea kinetic modeling (UKM) was found to correlate well with total body water (VKecn= 36.4 +/- 5.2 L). All anthropometric equations overestimated measured V: VWatson= 40.7 +/- 3.9 L, VHume= 41.8 +/- 2.5 L, VChertow= 44.6 +/- 3.3 L, and VChumlea= 43.1 +/- 2.9 L. Single-pool Kt/V obtained by kinetic modeling was used as reference (Kt/V)SPVV= 1.49 +/- 0.15. Using different Vs as the V component in the ionic dialysance Kt/V, we obtained: Kecn*t/VWatson= 1.34 +/- 0.12, Kecn*t/VBIS= 1.51 +/- 0.21 and Kecn*t/VKecn= 1.52 +/- 0.18.

The single-pool Kt/V calculated using the ionic dialysance method agreed with the conventional blood sample method provided that V was calculated using BIS or urea kinetics. V by either method was reproducible and varied little in an individual patient. Monthly determination of V allows determination of Kt/V for each dialysis session by ionic dialysance.