[Membrane biocompatibility in dialysis: the role of absorption]

Membrane biocompatibility is a concept that have gained clinical relevance. How to define a "biocompatible membrane" in hemodialysis is still object of discussion. Intermediate biochemical reactions, measured in the blood are more relevant than clinical events to document membrane's quality. In the absence of prospective studies, it is not possible to document that the constant use of a dialysis membrane governs risk of death in the hemodialyzed patient. Nevertheless, some clinical criteria are relevant, such as hemocompatibility, i.e. clotting of the extracorporeal circuit and heparin consumption, hypersensitivity reactions, denutrition associated with the "microinflammatory" stress induced by the hemodialysis session, occurrence of beta 2 microglobulin-derived amyloidosis. Synthetic membranes are credited of higher biocompatibility than cellulosic membranes. In general, they are highly permeable to peptides and proteins of the middle molecular range that contain some uremic toxins. In addition to be "highly permeable", allowing convective transfert, some synthetic membranes (polymethylmetacrylate, polyacrylonitrile, polyamide) bind proteins. Protein adsorption into synthetic membrane results from electrical charges distribution both at the surface and in the bulk of the membrane. Ionic interactions are the main contribution to protein adsorption, but rheological conditions, surface rugosity, and porosity or gel consistency play also a role. Consequently, some membranes can bind cytokines and oxygen species, other bind endotoxins. Recently, it has been demonstrated that heparin coating was possible with the AN69-ST membrane, resulting in improved hemocompatibility and significant lessening of heparin requirements during the sessions. It appears that adsorption characteristics govern biocompatibility. For clinical practice, a classification of various membranes according to these properties must be taken into account.