Hemodialysis is a life-sustaining treatment for individuals whose kidneys are no longer able to filter the blood. This process requires a specialized medical device, technically known as a dialyzer or artificial kidney. The dialyzer is a cylindrical filter containing thousands of microscopic, hollow fibers made of a semi-permeable membrane. As a patient’s blood flows through these fibers, a cleansing solution, or dialysate, flows around the outside. Waste products and excess fluid move across the membrane into the dialysate, allowing the cleansed blood to be returned to the body. Dialyzer types are distinguished by the filtration properties of their membranes, which accommodate waste molecules of different sizes.
Defining Dialyzer Flux
The term “flux” refers to the membrane’s permeability, specifically its ability to transport water and solutes across its surface. A low flux dialyzer has lower hydraulic permeability, which restricts the movement of fluid and certain waste molecules. This low permeability is quantified by the ultrafiltration coefficient (Kuf), typically defined by a Kuf value less than 12 to 15 milliliters of fluid per hour per millimeter of mercury (mL/h/mmHg).
Solute removal occurs primarily through diffusion and convection. Diffusion involves the movement of solutes from the blood (high concentration) to the dialysate (low concentration). Low flux dialyzers rely heavily on diffusion and effectively remove small molecules, such as urea, which weigh less than 500 Daltons.
Convection involves solutes being dragged across the membrane along with the fluid during ultrafiltration. Because low flux membranes have low hydraulic permeability, their ability to use convection to clear larger molecules is limited. This means that larger, middle-sized molecules are not efficiently cleared by low flux dialyzers. Compared to high flux dialyzers, low flux dialyzers are designed for less aggressive filtration.
Membrane Structure and Composition
The physical characteristics of the membrane material determine a dialyzer’s low flux classification. Historically, low flux membranes were often made from regenerated cellulose, such as Cuprophan, which had a small, uniform pore size. Pore size is the main structural feature dictating which solutes can pass through.
Low flux membranes have smaller pores that prevent the passage of larger molecules and beneficial proteins, such as albumin (approximately 66,000 Daltons). This design ensures albumin is retained in the blood. Many modern low flux dialyzers use synthetic polymers, such as polysulfone, modified to maintain a low Kuf value and the characteristic small pore size.
The ultrafiltration coefficient (Kuf) serves as the metric for the membrane’s hydraulic permeability. A low Kuf value confirms the low flux designation by corresponding directly to the membrane’s resistance to water flow. The membrane’s thickness and overall surface area also contribute to the final efficiency, but the pore size and Kuf are the defining factors for its classification.
When Low Flux Dialyzers Are Used
Low flux dialyzers are commonly used for standard, conventional hemodialysis treatments globally. They provide effective removal of small, water-soluble waste products like urea and creatinine, which is necessary for maintaining fluid and electrolyte balance in patients with kidney failure. In clinical practice, these dialyzers are often selected for patients who are generally stable or do not require the high toxin clearance offered by other membrane types.
A primary limitation is the poor clearance of middle molecules, specifically beta-2 microglobulin (\(\beta_2\)M). \(\beta_2\)M is a protein with a molecular weight of 11.8 kDa, and its accumulation is a recognized problem for long-term dialysis patients. Low flux dialyzers typically have a \(\beta_2\)M clearance rate less than 10 to 15 mL/min, which leads to high plasma levels of this protein.
The accumulation of \(\beta_2\)M is directly linked to dialysis-related amyloidosis, a condition where the protein forms deposits in bones, joints, and tendons. Studies show that patients treated solely with low flux dialyzers have significantly higher concentrations of \(\beta_2\)M compared to those using higher-flux options. Elevated \(\beta_2\)M levels are also identified as a predictor of higher long-term mortality.
Despite these limitations, low flux dialyzers minimize the risk of back-filtration, where impurities from the dialysate could cross the membrane into the blood. Their low permeability provides a barrier against this risk, making them a safe choice in settings where ultra-pure dialysate cannot be guaranteed. The decision involves a medical trade-off between proven safety regarding back-filtration and the recognized long-term risk associated with reduced middle molecule removal.