Ionic Dialysance Measurement Is Urea Distribution Volume Dependent: A New Approach to Better Results

Conductivity (CD)-based dialysance measurements precisely match urea dialysance with <5% difference. For measurement, a CD step-profile is applied by increasing dialysate inlet CD at time to for 10% above baseline and lasting for 2-5 min until t 1 , followed by a decrease to -4% until t 2 and a final return to baseline, meanwhile recording dialysate CD at filter inlet (cdi) and outlet (cdo), dialysate flow (Qd), and ultrafiltration (UF)-rate (Qf). Electrolytic dialysance (KeCn) is calculated by KeCn I , J = (1 - [cdo I - cdo J ]/[cdi I - cdi J ])(Qd + Qf) with time index I≠J. The combinations in I,J are not equivalent: KeCn 0 , 1 < KeCn 1 , 2 < KeCn 0 , 2 . Each difference is 2% to 5%, and a difference versus urea clearance remains. An in vivo on-line clearance study (10 patients, 100 dialysis sessions, 265 measurements) with automatic electrolytical dialysance measurements and permanent data recording was conducted. Two methods were applied: a CD step-profile and a significantly smaller, dynamic CD bolus. Both were compared to laboratory reference of urea clearance. Reference Kt/V has been calculated using equilibrated single-pool methods and direct quantification. Urea generation was ignored. The results are as follows. The reference blood-side urea clearance was 164.0 ′ 11.8 ml/ min, n = 265. The mean errors of the ionic dialysance results are KeCn 0 , 1 : -9.1 ′ 4.8%, n = 250; KeCn 1 , 2 : -5.6 ′ 4.4%, n = 250; KeCn 0 , 2 : 6.8 ′ 7.7%, n = 250; KeCn B o l u s : 0.1 ′ 4.8%, n = 162. The KeCn I , J error is urea distribution volume related. Kt/V comparison to equilibrated single pool is as follows: KeCn 1 , 2 t/V: 0.0 ′ 5.0% (r = 0.96, n = 45); KeCn B o l u s t/V: 5.3 ′ 3.9% (r = 0.98, n = 44). The comparison to direct quantification is as follows: KeCn 1 , 2 t/ V: -2 ′ 6.4% (r = 0.95, n = 68); KeCn B o l u s t/V: 3.2 ′ 6.3% (r = 0.95, n = 66). V could roughly be measured. Dialysance measured by the step-function method was dependent on sodium load and distribution volume while the CD-bolus dialysance was not. Errors are generated by measurement-induced sodium shift that is sufficient even to estimate urea distribution volume. For dialysance measurements, small dynamic CD boli are preferable to stable step functions.