Net ultrafiltration may not eliminate backfiltration during hemodialysis with highly permeable membranes

Backfiltration of dialysis solution can occur during hemodialysis with highly permeable membranes. A method has recently been developed for determining backfiltration rates in vitro at low dialysate flow rates by measuring changes in the local dialysate concentration of a marker macromolecule via sampling ports added to the hemodialyzer housing. In the present study, the influence of net ultrafiltration on backfiltration rates was determined for five commercial dialyzers containing membranes with different water permeabilities. In vitro experiments were performed (n = 3) using freshly donated whole blood at blood flow rates of 200 and 340 ml/min and at a dialysate flow rate of 100 ml/min. At zero net ultrafiltration, backfiltration rates increased with increasing membrane water permeability and ranged from 0.9 to 6.9 ml/min. At a net ultrafiltration rate of 10 ml/min, backfiltration was eliminated for dialyzers containing membranes with water permeabilities of less than 30 ml/h/mm Hg but remained significant for dialyzers with higher membrane water permeabilities. Therefore, despite a significant net ultrafiltration rate, backfiltration may still occur during hemodialysis with highly permeable membranes.