Editor’s note: This article is an English translation of the original German article, “Dialyse-Spezialwissen: Intradialytische Prozesse,” written by Gerd Breuch and originally published in the fnb eJournal, Issue 03/26. The original German version is available here. This English translation is republished with permission from the author and the original publisher.
This article addresses the intradialytic process of backfiltration. It explains what backfiltration is, how it develops in the dialyser and its relevance to dialysis therapy. It also outlines the requirements – particularly regarding dialysis fluid – for its safe use.
Keywords: Backfiltration, transmembrane pressure, high-flux dialysis, ultrafiltration, internal filtration, convective solute transport, ultrapure dialysate, dialysis.
What Is Backfiltration?
Backfiltration is a normal physical process treatments using high-flux dialysers. that occurs particularly during dialysis In the front section of the dialyser (blood inlet), the hydrostatic pressure in the blood compartment (inside the hollow fibres) is higher than in the dialysate compartment. This pressure difference causes water to pass from the blood compartment through the semipermeable hollow-fibre membrane into the dialysate compartment (see Fig. 2).
Hydrostatic pressure describes the pressure exerted by a fluid on its surroundings. In the dialyser, this pressure is created in both compartments by the respective fluid flows. On the blood side, it is mainly generated by the blood pump, which drives the blood through the fine hollow fibres. On the dialysate side, pressure is determined by the dialysate flow and the settings of the dialysate circuit.
As blood travels through the hollow fibres of the dialyser, hydrostatic pressure in the blood compartment continuously decreases because the fine fibres create resistance to blood flow. Pressure in the dialysate compartment, by contrast, changes only slightly. This reduces the pressure difference between the blood and dialysate compartments, meaning that progressively less fluid passes from the blood through the membrane into the dialysate along the length of the dialyser.
At a certain point, hydrostatic pressure on the blood and dialysate sides becomes equal. Beyond this point, hydrostatic pressure in the blood compartment is lower than in the dialysate compartment. Consequently, the direction of water flow reverses: dialysate now flows back through the membrane into the blood. This process is referred to as backfiltration or internal filtration. The volume of water over-filtered from the blood in the front section of the dialyser, in addition to the set net ultrafiltration, is thereby returned to the blood.
Backfiltration therefore does not alter net ultrafiltration (the prescribed ultrafiltration volume), but only the distribution of fluid movements within the dialyser.
Modern dialysis machines operate with volumetric control. They continuously measure and regulate dialysate inflow and outflow, ensuring that exactly the fluid volume prescribed as ultrafiltration is removed from the system, despite internal filtration.
In the front section of the dialyser, more water is filtered from the blood than is ultimately intended to be removed from the patient. Part of this filtered volume returns to the blood as backfiltration in the rear section.
Is backfiltration the same as haemofiltration?
The principle of internal filtration (backfiltration) is based on the same physical principle as haemodiafiltration (HDF). Filtration followed by fluid replacement creates convective solute transport. In HDF, fluid replacement is provided by the defined substitution volume or the flow of the substitution pump. In internal filtration, however, it is achieved through the backfiltration of dialysate across the membrane.
Unlike HDF, filtration and backfiltration volumes are not set intentionally; they result from pressure conditions inside the dialyser. Therefore, they cannot be quantified directly. In addition, internal filtration and backfiltration volumes are considerably lower than the filtration and substitution volumes achieved in conventional HDF.
The safe use of internal filtration requires ultrapure dialysate, since dialysate passes through the membrane into the bloodstream during backfiltration. When ultrapure dialysate is used, however, this process is not an undesirable side effect but a beneficial component of the dialysis procedure.
The increased convective solute transport particularly improves the removal of middle-molecular-weight uraemic toxins, whose elimination by diffusion is less efficient due to their molecular size. Although internally filtered and backfiltered fluid volumes are significantly lower than in haemodiafiltration, they still make a measurable contribution to convective solute removal and therefore support effective dialysis therapy.
Special HX dialysers specifically promote backfiltration through their medium cutoff (MCO) membrane and a membrane design tailored to enable increased internal filtration. MCO membranes feature a defined, narrow pore-size distribution and greater permeability for medium-sized molecules. This improves convective removal of middle-molecular-weight uraemic toxins without relevant albumin loss.
Summary
Backfiltration is a physiological and desirable process in haemodialysis with highflux dialysers. It is caused by changing transmembrane pressure conditions along the dialyser and leads to internal filtration followed by the backfiltration of ultrapure dialysate into the blood.
The prescribed net ultrafiltration remains unchanged because modern volumetrically controlled dialysis machines precisely regulate fluid removal. Although the fluid volumes involved are lower than in haemodiafiltration, internal filtration contributes to improved elimination – particularly of middle-molecularweight uraemic toxins – through additional convective solute transport.
References
Breuch, G., Müller, E. (2025). Specialist Nursing in Nephrology and Dialysis (7th edition). Elsevier.
Breuch, G., & Servos, W. (2017). Dialysis for Beginners (4th edition). Elsevier.
