A reliable method to assess the water permeability of a dialysis system: the global ultrafiltration coefficient*

Background: Recent randomized controlled trials suggest that sufficiently high convection post-dilutional haemodiafiltration (HC-HDF) improves survival in dialysis patients, consequently this technique is increasingly being adopted. However, when performing HC-HDF, rigorous control systems of the ultrafiltration setting are required. Assessing the global ultrafiltration coefficient of the dialysis system [(G)K(D-UF); defined as ultrafiltration rate (Q(UF))/transmembrane pressure] or water permeability may be adapted to the present dialysis settings and be of value in clinics. Methods: (G)K(D-UF) was determined and its reproducibility, variability and influencing factors were specifically assessed in 15 stable patients routinely treated by high-flux haemodialysis or HC-HDF in a single unit. Results: (G)K(D-UF) invariably followed a parabolic function with increasing Q(UF) in dialysis and both pre- and post-dilution HC-HDF (R(2) constantly >0.96). The vertex of the parabola, (G)K(D-UF)-max and related Q(UF) were very reproducible per patient (coefficient of variation 3.9 ± 0.6 and 3.3 ± 0.3%, respectively) and they greatly varied across patients (31–42 mL/h(−1)/mmHg and 82–100 mL/min, respectively). (G)K(D-UF)-max and its associated Q(UF) decreased during dialysis treatment (P < 0.01). The (G)K(D-UF)-max decrease was related to weight loss (R(2) = 0.66; P = 0.0015). Conclusions: (G)K(D-UF) is a reliable and accurate method to assess the water permeability of a system in vivo. It varies according to dialysis setting and patient-related factors. It is an objective parameter evaluating the forces driving convection and identifies any diversion of the system during the treatment procedure. It is applicable to low- or high-flux dialysis as well as pre- or post-dilution HDF. Thus, it may be used to describe the characteristics of a dialysis system, is suitable for clinical use and may be of help for personalized prescription.