Membrane requirements for high-flux and convective therapies

Worldwide, high-flux dialysis (HF-HD) has now surpassed low-flux dialysis (LF-HD) as the predominant treatment modality, recognition that removal of larger uremic retention solutes is desirable for the treatment of patients with end-stage chronic kidney disease (CKD). An even more advanced form of HF-HD in terms of removal of a broad spectrum of uremic toxins is on-line hemodiafiltration (HDF), involving convective transport mechanisms for solute removal. With the modality reaching considerable technical maturity over the last two decades, on-line HDF is now recognized for its clinical efficiency and effectiveness, versatility and safety. Such has been the success of on-line HDF that, in Europe, more patients are treated with on-line HDF than even peritoneal dialysis. Fabrication of high-flux membranes for convective therapies is more than a matter of simply making the membrane 'more open' or of increasing the membrane pore size which is not the only determinant for achieving higher convection. While convective transport of larger uremic retention solutes primarily demands membranes with high hydraulic permeability and sieving capabilities, the making of a modern dialysis membrane involves several other considerations that culminate in the delivery of an effective and safe therapy. In this communication I outline the essential membrane requirements and principles for solute removal by convection, as well of meeting additional features related to the therapy. The basic principles of the membrane manufacturing processes by which desired membrane morphology is derived for the separation phenomena involved in dialysis are further described. An awareness of this enables one to appreciate that, depending on the individual constituents and variations of the manufacturing processes, fabrication of all high-flux membranes entails achieving a balance between the ideal or desired criteria for blood purification. Dialysis membranes for convective therapies, even from the same base polymer, exhibit significant differences in their morphology and thus in their ability to facilitate convection.