Daily Dialysis and Flexible Schedules: How to Assess Kt/V and EKR_c ?

Despite the growing interest in daily hemodialysis (DHD), logistic and economic factors limit its dissemination. Not the least of these factors is the lack of uniform criteria for measuring efficiency. From November 1998 to November 2000, 19 patients were on DHD in our unit. The dialysis prescription was bicarbonate buffer; 6 sessions per week; 2 - 3 hours; blood flow 250 - 350 mL/min; individual K, HCO₃ , and Na levels; membrane 1.6 - 2 m² (polysulfone, polycarbonate). The prescription represented the minimum dialysis requirement; patients were free to add up to 30 minutes per session, further increase or any decreases needed confirmation by the caregivers. The aim of the study was to assess Kt/V_urea variability in this clinical setting, and to identify the minimum number of dialysis sessions required to obtain a reliable estimate of weekly Kt/V_urea [relative error (RE) < 10%]. We studied 169 dialysis sessions in 13 clinically stable patients on DHD for ≥ 3 months, with ≥ 3 Kt/V_urea measurements within 2 weeks (median: 10; range: 3 - 32 sessions), tested in the same laboratory. To assess variability, we employed the simplest formula (the Lowrie Kt/V_urea ), the widely used Daugirdas II formula, and the derived single-pool equivalent renal clearance (EKR_c ), according to Casino. The variability of Kt/V_urea per session was high (Lowrie: RE = 2.5% - 22.1%; Daugirdas II and EKR_c : RE = 3.6% - 24%). Averaging several dialysis sessions leads to a more reliable estimate of weekly efficiency (6 sessions: RE = 0; 3 sessions, Lowrie formula: Kt/V_urea RE = 1.1% - 9.7%; Daugirdas II and EKR_c : RE = 1.6% - 10.6%). In patients with wide time variations, variability may be lower if weekly efficiency is determined on the basis of "average hourly Kt/V_urea ," which is calculated by dividing Kt/V_urea by the number of hours in the studied sessions, and then multiplying by the hours of dialysis performed in the whole week (Lowrie formula, Kt/V_urea : RE = 4.8% - 16.6% for 1 session, 2.1% - 7.3% for 3 sessions). Once again, the RE decreases sharply when data from 3 sessions are considered. Therefore, for flexible DHD, we suggest averaging the data from ≥ 3 sessions for weekly Kt/V_urea assessment.