There has been a lot of attention these days regarding increasing “middle molecule clearance”, a topic already covered previously here. Certainly it is well-established that buildup of beta-2-microglobulin can lead to a dialysis-derived form of amyloidosis, and it is further speculated that clearance of other “evil humours” of larger molecular weight might have a benefit as well. There are a couple different ways to achieve improved middle molecule clearance: HIGH FLUX DIALYSIS and HEMOFILTRATION. What’s the difference between the two?
High flux dialysis refers to using a type of dialysis membrane (these days in the form of many thousand microfibers) which has a larger pore size than older, “low-flux” dialyzers. The mechanism of solute removal still relies no diffusion, however, rather than convection. Although most dialysis centers have transitioned to using “high-flux” dialyzers due to a belief that they are inherently more efficient than their low-flux counterparts, the largest randomized controlled trial looking at high-flux versus low-flux dialyzers (the HEMO Study) failed to show a clinically meaningful benefit of high-flux dialyzers.
Hemofiltration can also result in improved middle molecular clearance; for the same time, blood flow rate, and pore size, hemofiltration improves middle molecule clearance when compared to hemodialysis. The driving force for solute clearance in hemofiltration is hydraulic pressure and the convective clearance which results. Furthermore, some view hemofiltration as being more “natural” than hemodialysis in the sense that the glomerulus works primarily by generation of a pressure gradient.
A recent study (Santoro et al, AJKD 2008) which randomized ESRD patients to either hemofiltration or hemodialysis showed a survival benefit for hemofiltration. However it is notable that the sample size was small, and that the hemodialysis in this study was using low-flux dialyzers. In my opinion, if the authors are trying to prove the superiority of hemofiltration to hemodialysis, they should include high-flux dialyzers in the dialysis arm, as that appears to be becoming standard-of-care.