Dialysis filters waste products, excess fluid, and surplus electrolytes from the blood when your kidneys can no longer do the job. The most familiar targets are small waste molecules like urea and creatinine, but the full list is much longer and more complex. Dozens of substances move across the dialysis membrane during a session, some intentionally removed and some lost as collateral damage.
Small Waste Molecules
The headline substances that dialysis is designed to clear are small, water-soluble waste products your body generates through normal metabolism. Urea, a byproduct of protein breakdown, is the most tracked marker of dialysis effectiveness. Creatinine, produced by muscle metabolism, is another standard target. These molecules are small enough to pass easily through dialysis membranes by diffusion, moving from the higher concentration in your blood to the lower concentration in the dialysate fluid on the other side of the membrane.
In practice, dialysis machines clear urea at roughly 230 mL/min and creatinine at about 164 mL/min, and the two track well together as measures of how effectively a session is going.1PubMed Central. Creatinine generation from kinetic modeling with or without postdialysis serum creatinine measurement: results from the HEMO study Other small molecules cleared alongside them include uric acid (from purine metabolism) and various organic acids. Because these substances dissolve freely in blood plasma and are not attached to larger proteins, standard dialysis membranes handle them well. This is where conventional hemodialysis does its best work.
Middle Molecules and Why They Are Harder to Remove
Not all the harmful substances in kidney failure are small. A category researchers call “middle molecules” includes larger compounds that healthy kidneys would normally filter and degrade. The most studied example is beta-2 microglobulin (B2M), a protein fragment that accumulates dramatically in people on long-term dialysis. In end-stage kidney disease, B2M levels can climb to as much as 60 times normal.2International Journal of Nephrology and Renovascular Disease. Dialysis-related amyloidosis: challenges and solutions
When B2M builds up over years, it can form amyloid deposits in joints and bones, a condition called dialysis-related amyloidosis that causes pain, stiffness, and functional impairment.3PubMed Central. Beta-2 Microglobulin Amyloidosis: Past, Present, and Future Older cellulose-based dialysis membranes were poor at clearing B2M. More modern high-flux membranes, made from synthetic materials, remove and adsorb B2M far more effectively.4PubMed. Beta 2-microglobulin amyloidosis in chronic dialysis patients: a case report and review of the literature This is one of the clearest examples of how membrane technology directly shapes what dialysis can and cannot pull from the blood.
Protein-Bound Toxins, the Stubborn Category
Some of the most harmful uremic toxins ride through the bloodstream attached to albumin, and that attachment makes them extremely difficult to remove. The two most studied examples are p-cresyl sulfate and indoxyl sulfate, both produced when gut bacteria break down dietary amino acids. Because these toxins are tightly bound to albumin, a large protein that dialysis membranes are designed to keep in the blood, only the tiny unbound fraction is available for filtration. Standard hemodialysis fails to clear them efficiently.5PubMed Central. Removal of Protein-Bound Uremic Toxins during Hemodialysis Using a Binding Competitor
This is a real limitation. These protein-bound toxins are linked to cardiovascular damage, immune dysfunction, and progression of kidney disease, yet conventional dialysis barely dents their levels. Researchers have explored adding binding competitors to the dialysate, essentially molecules that knock the toxins loose from albumin so they can cross the membrane. Others are developing specialized adsorbent materials designed to strip these toxins away.6PubMed. Adsorption Removal of Protein-Bound Uremic Toxins: Material Strategies, Dissociation Mechanisms, and Clinical Challenges Neither approach has become standard clinical practice yet, so protein-bound toxins remain the category where dialysis falls shortest.
Electrolytes and Acid-Base Correction
Beyond waste products, dialysis adjusts the chemical balance of your blood. When kidneys fail, potassium tends to accumulate, and high potassium is dangerous because it can disrupt heart rhythm. During a session, potassium moves from your blood into the dialysate, which is formulated with a deliberately low potassium concentration to create the gradient that drives removal. Phosphorus, which builds up and contributes to bone disease and vascular calcification in kidney failure, is also cleared during treatment.
At the same time, dialysis corrects the metabolic acidosis that accompanies kidney failure. Bicarbonate in the dialysate diffuses into the blood, buffering the accumulated acid. The interplay between bicarbonate transfer and potassium removal is more nuanced than it might seem. Higher bicarbonate concentrations in the dialysate cause sharper drops in blood potassium levels during treatment, not because more potassium is actually removed, but because the rising blood pH pushes potassium into cells faster.7PubMed Central. Dialysate Potassium Concentration: Should Mass Balance Trump Electrophysiology? The total potassium leaving the body may be similar, but the rapid intracellular shift can still cause problems, including arrhythmias during the session itself. Dialysis teams calibrate potassium and bicarbonate concentrations in the dialysate carefully to balance effective correction against the risk of too-rapid changes.
Sodium and calcium are also exchanged across the membrane, with concentrations in the dialysate chosen to maintain or gently correct blood levels. The dialysate is essentially a custom recipe designed to pull out what you have too much of and add back what you need.
Excess Fluid
Fluid removal is a separate but equally critical function. Between dialysis sessions, most patients accumulate fluid that their kidneys can no longer excrete. This shows up as swelling, elevated blood pressure, and strain on the heart. During treatment, the dialysis machine creates a pressure difference across the membrane that drives water from the blood side to the dialysate side, a process called ultrafiltration.8PubMed. Ultrafiltration and backfiltration during hemodialysis
How much fluid gets removed depends on how much weight you have gained since the last session. Typical ultrafiltration targets range from one to four liters over a three- to four-hour treatment. Removing too much too quickly can cause blood pressure to drop, cramping, and dizziness. Too little removal leaves you fluid-overloaded and at higher cardiovascular risk. Hitting the right balance session after session is one of the ongoing challenges of life on dialysis.
What Dialysis Removes That You Actually Need
The dialysis membrane does not distinguish between harmful waste and useful small molecules. If a substance is small and water-soluble, it will diffuse across the membrane regardless of whether you want it gone. This means each session strips away nutrients along with the toxins.
Amino acids are the most studied unintended loss. Patients lose roughly 12 grams of amino acids per session through the dialysate, including about 3.7 grams of essential amino acids that the body cannot synthesize on its own. Blood levels of total and essential amino acids drop measurably over the course of a treatment.9PubMed Central. End-Stage Renal Disease Patients Lose a Substantial Amount of Amino Acids during Hemodialysis Over months and years, this repeated loss contributes to muscle wasting and protein-energy malnutrition, which are common in long-term dialysis patients.
Water-soluble vitamins, particularly B vitamins and vitamin C, are also lost during treatment. Small amounts of glucose leave as well.10PubMed. Nutritional aspects in hemodialysis This is why dialysis patients are typically prescribed daily water-soluble vitamin supplements and are encouraged to eat higher-protein diets than might otherwise be recommended. The dietary advice for someone on dialysis often seems contradictory: eat more protein to compensate for amino acid losses, but watch your potassium and phosphorus intake because those accumulate between sessions. Managing those competing demands is a daily reality for people on long-term treatment.
Medications Getting Caught in the Filter
If dialysis removes small, water-soluble molecules, it follows that many medications with those characteristics get filtered out too. Whether a drug is cleared during a session depends on several properties: its molecular weight, how tightly it binds to blood proteins, and how widely it distributes into body tissues.11PubMed. Drug Dosing in Postdilution Hemodiafiltration: Pharmacokinetic Principles and Clinical Implications A small drug that floats freely in plasma gets stripped away efficiently. A large drug bound tightly to albumin and distributed deep into tissues barely budges.
This matters because it affects when you take your medications and whether you need an extra dose after treatment. Antibiotics are a common example: some are so efficiently removed that patients need a supplemental dose after each session to maintain therapeutic levels. Others are barely affected. The membrane type and the mode of dialysis also play a role, since high-flux membranes and hemodiafiltration clear a broader range of molecular sizes than conventional setups. Your nephrologist and pharmacist coordinate dosing schedules around your dialysis sessions for exactly this reason.
How Hemodiafiltration Expands the Range
Standard hemodialysis relies primarily on diffusion: substances move across the membrane from high concentration to low concentration. This works well for small molecules but is limited for larger ones. Hemodiafiltration (HDF) adds a second removal mechanism called convection, where fluid is actively pushed through the membrane and carries dissolved solutes of varying sizes along with it, then replacement fluid is infused back into the blood.12PubMed Central. Uremic Toxins and Hemodiafiltration: From Molecular Mechanisms to Clinical Outcomes
The practical effect is that HDF clears middle molecules like B2M more effectively than standard hemodialysis.13PubMed Central. Comparative Analysis of Hemodiafiltration Methods: Efficacy and Safety in Maintenance Hemodialysis The volume of fluid exchanged during the convective process (convection volume) directly influences how well middle molecules are removed: more fluid exchanged means better clearance.14Lékař a technika – Clinician and Technology. OPTIMIZED CONVECTIVE VOLUME IN ONLINE HEMODIAFILTRATION Different configurations (pre-dilution, post-dilution, and mixed modes) each have trade-offs in how effectively they clear different size ranges. Post-dilution HDF tends to be most efficient for middle molecules, while mixed modes offer a compromise between performance and practical ease of delivery.
HDF has become increasingly common, particularly in Europe and parts of Asia, and some large trials have suggested survival benefits over conventional hemodialysis, though the evidence is still being debated. What is clear is that HDF gives clinicians a way to target a broader spectrum of toxins than diffusion alone can handle.
What Dialysis Purity Means for Inflammation
Dialysis does not just remove substances from the blood. In a less intuitive twist, it can also introduce unwanted material. The dialysate fluid that runs on the other side of the membrane is made from treated water, and if that water contains bacterial fragments or endotoxins, those can cross the membrane into the patient’s bloodstream. Even fragments too small to be whole bacteria can stimulate immune cells and drive chronic inflammation.15PubMed. Ultrapure dialysate
This is why ultrapure dialysate, water that meets stricter standards for bacterial and endotoxin contamination, has become the standard of care in many dialysis centers. Patients treated with ultrapure dialysate show lower levels of inflammatory markers, including reduced release of pro-inflammatory cytokines like TNF-alpha and IL-6.16PubMed. On-line production of ultrapure substitution fluid reduces TNF-alpha- and IL-6 release in patients on hemodiafiltration therapy For patients on hemodiafiltration, where large volumes of replacement fluid are infused directly into the bloodstream, the purity of that fluid matters even more. Chronic low-grade inflammation is a major driver of cardiovascular disease in dialysis patients, so reducing it through cleaner fluid is a meaningful clinical gain even though it has nothing to do with what gets filtered out.
Why Residual Kidney Function Changes the Picture
If you still have some native kidney function when you start dialysis, even a small amount, it substantially changes the equation. Residual kidney function provides continuous clearance around the clock rather than the intermittent three-times-a-week schedule of standard hemodialysis. And it handles the substances that dialysis struggles with most: middle molecules and protein-bound toxins both rely heavily on residual kidney function for clearance.17PubMed Central. Advances in Understanding and Management of Residual Renal Function in Patients with Chronic Kidney Disease
Preserving whatever residual function remains is associated with better survival and quality of life, making it a priority in dialysis management.18PubMed Central. Preservation of residual kidney function in hemodialysis patients: reviving an old concept Certain practices help protect it: avoiding large swings in blood pressure during sessions, limiting nephrotoxic medications, and considering gentler dialysis schedules in early treatment. Peritoneal dialysis, which uses the lining of the abdomen as a filter, tends to preserve residual function longer than hemodialysis, though it has its own clearance trade-offs. For example, peritoneal dialysis removes some beneficial antioxidant compounds like ergothioneine less efficiently per session than hemodialysis does.19PubMed Central. Ergothioneine Depletion in Peritoneal Dialysis
The broader point is that dialysis is not a binary on-or-off replacement for kidney function. It handles some tasks well (small waste clearance, fluid removal, electrolyte correction) and others poorly (protein-bound toxin removal, continuous around-the-clock clearance). Whatever your own kidneys can still contribute fills gaps that the machine cannot.
Membrane Materials and Ongoing Research
The membrane itself is arguably the most important variable determining what dialysis can filter. Early cellulose membranes were relatively unselective and provoked inflammatory reactions in the blood. Modern synthetic membranes made from materials like polysulfone are more biocompatible and allow better passage of middle molecules while keeping albumin and blood cells on the right side of the barrier.
Research into next-generation membranes is active. One example involves loading polysulfone membranes with titanium dioxide and hydroxyapatite composites. In testing, these modified membranes achieved urea clearance above 73% and creatinine clearance above 72%, while reducing unwanted protein adsorption by about a third and keeping blood cell destruction (hemolysis) below 5%.20Artificial Organs. Fabrication and characterization of TiO2‐hydroxyapatite composite‐loaded Polysulfone membranes with integrated biocompatibility for dialysis application The goal is membranes that clear more toxins, cause less inflammation, and do less collateral damage to blood components.
Other lines of research focus on sorbent-based approaches, where adsorbent materials are incorporated into the dialysis circuit to specifically capture protein-bound toxins that membranes alone cannot handle. None of these technologies has replaced the current standard, but they reflect an active recognition that what dialysis filters out today is not the complete list of what it should be filtering out. The gap between what healthy kidneys remove and what a machine removes remains wide, and closing it even partially would meaningfully improve outcomes for the millions of people worldwide who depend on this treatment.