Dialyser membranes: low-flux, high-flux, and high cut-off

When is a dialysis membrane called low-flux or high-flux?

Dialysis membranes are traditionally divided into low-flux and high-flux membranes mainly according to:

1. The ultrafiltration coefficient, KUF.

2. The ability to remove middle molecules, particularly beta-2 microglobulin.

3. Membrane permeability, pore-size distribution, and sieving characteristics.

There is no single numerical threshold that universally and completely separates every low-flux membrane from every high-flux membrane. Definitions vary between publications, standards, and manufacturers.

Traditional comparison

Why does KUF matter?

KUF, the ultrafiltration coefficient, describes how readily water crosses the dialysis membrane under a pressure gradient.

A higher KUF indicates greater hydraulic permeability and can support a larger convective transport component.

KUF alone cannot fully define a high-flux membrane. High water permeability does not guarantee identical permeability to larger solutes; pore size, pore distribution, membrane thickness, adsorption, and protein interactions also matter.

Why use beta-2 microglobulin?

Beta-2 microglobulin has a molecular mass of approximately 11.8 kDa and is commonly used as a representative middle molecule when membrane performance is assessed.

Its removal is very limited with low-flux HD, whereas high-flux membranes permit substantially greater clearance.

Modern characterization therefore considers not only KUF but also:

• Beta-2 microglobulin clearance.

• The sieving coefficient.

• Membrane pore size and distribution.

• Permeability to middle molecules and retention of essential proteins.

An important distinction: high-flux is not HDF

These terms describe different things.

High-flux is a property of the dialyser membrane.

Haemodiafiltration (HDF) is a treatment technique that intentionally combines:

Diffusion + convection

A high-flux membrane can therefore be used for ordinary high-flux HD without the treatment being HDF.

A simplified comparison

Low-flux HD

Small molecules → cross readily
Middle molecules → passage is limited

↓

High-flux HD

Small molecules → cross readily
Middle molecules → cross more effectively, including beta-2 microglobulin

↓

HDF

A highly permeable membrane is used with an intentionally large convective volume, improving the removal of eligible middle molecules.

What about high cut-off membranes?

Membranes with larger effective pores are called high cut-off (HCO) membranes.

Conceptually, permeability increases along this sequence:

Low-flux → high-flux → high cut-off

Moving to the right permits passage of progressively larger molecules. With very permeable membranes, however, the risk of losing important molecules such as albumin also rises. HCO therapy is therefore a specific clinical intervention, not simply “better dialysis”.

Conclusion

It is not sufficient to define high-flux only as:

KUF >20 = high-flux

That is a useful traditional simplification, not an absolute universal definition.

A more accurate description is that a high-flux dialyser has high hydraulic permeability and a demonstrated ability to remove selected middle molecules such as beta-2 microglobulin, while such removal remains limited with a low-flux dialyser. The chosen membrane must also retain essential proteins, be biocompatible, and fit the overall prescription.