In vitro assessment of dialysis membrane as an endotoxin transfer barrier: geometry, morphology, and permeability

High-flux dialysis membranes used with bicarbonate dialysis fluid increase the risk of back diffusion of bacterial endotoxin into the blood during hemodialysis. Endotoxin transfer of various synthetic fiber membranes was tested with bacterial culture filtrates using an in vitro system testing both diffusive and convective conditions. Membranes were tested in a simulated dialysis mode with endotoxin challenge material (approximately 420 EU/mL) added to the dialysis fluid, with saline used to model both blood and dialysis fluid. Samples were taken of both blood and dialysis fluid, and analyzed using a kinetic turbidimetric Limulus amoebocyte lysate assay. Endotoxin was found in all of the blood circuit samples, except for the Fresenius Optiflux F200NR(e) and thick-wall membranes. All membranes tested removed approximately 95% of the endotoxin from solution, with the residual approximately 5% recirculating within the dialysis fluid compartment. Endotoxin distribution through the fiber membrane was examined using a fluorescent-labeled endotoxin conjugate. Fluorescence images indicate that adsorption occurs throughout the membrane wall, with the greatest concentration of endotoxin located at the inner lumen. Contact angle analysis was able to show that all membranes exhibit a more hydrophilic lumen and a more hydrophobic outer surface except for the polyethersulfone membranes, which were of equal hydrophobicity. Resulting data indicate that fiber geometry plays an important role in the ability of the membrane to inhibit endotoxin transfer, and that both adsorption and filtration are methods by which endotoxin is retained and removed from the dialysis fluid circuit.