Effect of the ultrafiltration rate on solute removal efficiency in pre-dilution on-line hemodiafiltration

Pre-dilution on-line hemodiafiltration (HDF) is a blood purification therapy where solute removal is performed by both diffusion and convection, influenced by operational parameters. This study aimed to investigate the effects of ultrafiltration rate (Q_F) or alternately the substitution fluid flow rate (Q_S) and the membrane surface area of a diafilter on solute removal efficiency for solutes of varying molecular weights, using both experimental and theoretical analysis, and to clarify the mechanism of solute removal in this treatment. Experiments were conducted with three diafilters of different membrane surface areas (1.1, 1.5, and 3.0 m²) in aqueous and bovine blood systems, varying Q_F (=Q_S) from 0 to 400 mL/min. The parameters required for the theoretical calculation were determined by experiments. In aqueous system, clearances for three solutes (creatinine, vitamin B12, and inulin) obtained by the theoretical calculations corresponded well with the theoretical calculations, showing how the solute removal was depending on the diffusion and convection. In blood system, however, membrane fouling significantly impacted solute clearances, particularly in diafilters with smaller membrane surface areas or higher Q_F (=Q_S), leading to deviations between experimental and theoretical values. This study clarified the correlation between solute clearance and Q_F (=Q_S) with varying membrane surface area in pre-dilution on-line HDF and showed the importance of considering the effects of fouling on numerical models for enhanced clinical applicability.