High Flux or Low Flux?

I was asked this (paraphrased) question at Home Dialysis Central by a patient on dialysis in India:

I am on six nights a week, 7 hours each night home HD. I use F6 HPS dialysers. Is it a good idea to use these high flux dialysers or would regular F6 dialysers be good enough? Apart from being more expensive - in India, I pay out of pocket – can there be 'too much' solute removal ... like with phosphorus? With nocturnal dialysis, do high flux dialysers offer a benefit from additional removal of middle molecules and other solutes?

Good question!

In Australia, we use high flux membranes for almost all dialysis. That applies, not just for nocturnal HD, but for facility-based dialysis too – and regardless of the total delivered dialysis hours per week. Reuse was banned here in 1993 and most Australian units have used high flux dialysers since ~ 2000.

High flux dialysers are 'leakier' dialysers .. and that holds true for bi-directional membrane transit. This means that not only can more and larger solutes be removed from the patient—but at least potentially, more water-borne contaminants, e.g. endotoxin, can get in!

As a result, use of high flux dialysers demands additional in-line water protection. For our home patients, we use an added Diasafe™ filter between the personalised R/O unit and the machine. This is changed every 3 months, which adds a significant additional cost (~$A160/home patient/each 3 months), but we believe these filters are a non-negotiable accompaniment to use of a high flux dialyser.

While it is clearly arguable whether a high flux membrane is necessary in longer, slower, more frequent dialysis, there is evidence to show that B2 and homocysteine clearance does improve with a high flux membrane. We measure these routinely in all home NHD patients each 6 months. Early in our experience of nocturnal HD, we reported the clearance of β2 and homocysteine – and presumably other so-called 'middle molecules – to be markedly better with low-flux-but-long-hour, high frequency nocturnal HD compared to low-flux conventional 4-4.5 hour, 3 times weekly dialysis. So, the question remains: how much extra benefit accrues?

Sunny Eloot has performed some extremely elegant work in this area, using the Genius® System, and reported her results in KI in 2008(1). Her work compared the removal of larger molecular weight toxins from the deeper tissue compartments by a number of differing time and frequency models and confirmed the benefits of time and frequency. But, flux differences were not studied. It would be an interesting piece of work to use the same Genius System® to compare the influence of flux...

Cost clearly matters all over the world. Early data from Andreas Pierratos showed significantly greater β2 clearance with a low-flux membrane and with very low dialysate flow rates of 100ml/min. As removal of phosphate is so good with long, slow dialysis that many patients who exceed 30+ hour/week may need phosphate supplementation, it is likely that a low flux membrane is at the least a supportable proposition – at least for you in India, where cost issues become critical.

As far as I am aware, there have been no reported deficiency syndromes – apart from the excessive removal of phosphate – from 'over-dialysis'.

If high flux membranes and the addition (and extra cost) of 4 Diasafe®/year prove to be too costly, I doubt that a low-flux alternative would prove problematic. Low-flux but extended-hour dialysis is still far superior to conventional 12-15 hour facility-based dialysis, even if high-flux based. But, my view remains that a high flux membrane teamed with extra water filtration is the best option.

References

1. S Eloot, W Van Biesen, A Dhondt, H Van de Wynkele, G Glorieux, P Verdonck and R Vanholder. Impact of hemodialysis duration on the removal of uremic retention solutes Kidney International (2008) 73, 765–770