Can a healthy person book a dialysis session the way some people book a spa detox — walk out with cleaner blood and "rested" kidneys? The answer from a urologist at Mount Sinai's Icahn School of Medicine is a firm no, and the direction is exactly backwards. Dialysis is not a deep clean for the body. It is a rough artificial membrane standing in for kidneys that have already failed — and the rough substitute makes the part of the kidney still doing its job stop sooner.
Why the Machine Can't Copy the Kidney
A hemodialysis machine is a bundle of hollow fibers, and the fiber walls are full of tiny pores. Blood runs through the inside of the fibers while dialysate flows the opposite way outside. The two sides have different concentrations, so small molecules drift down the concentration gradient into the dialysate. Whether a waste molecule gets cleared comes down to two things: whether it is small enough, and whether it is free in the blood. Urea, creatinine, potassium and phosphorus are small and mostly unbound, so they clear well.
The kidney does not work that way. The glomerulus filters plasma first, and size matters there too — but the deciding step is the tubule. Tubule cells pull back water, glucose, bicarbonate and amino acids according to what the body needs right now, and at the same time they actively secrete: transporter proteins on the blood side (OAT1, OAT3 and their relatives) carry selected wastes into the cell, and a second set of transporters on the urine side pushes them into the tubule lumen. Both steps consume energy, and they keep working even when the blood concentration is already lower than the urine concentration.
That one move — actively pulling, not passively filtering — is what the membrane cannot do.
The Toxins Dialysis Misses
The difference shows up most clearly in a class of waste called protein-bound toxins.
Indoxyl sulfate and p-cresyl sulfate are small molecules, but they spend nearly their whole time in blood attached to albumin. Albumin is a large protein that cannot pass through the membrane pores. It flows past the membrane carrying its toxin passengers, and the passengers flow right past with it.
The tubule does not fight albumin for the toxin. It keeps removing the small free fraction in the blood; as the free concentration drops, more toxin dissociates from albumin to top it up, and the whole batch is slowly emptied. The dialysis membrane has no such continuous removal. Blood sits beside the membrane for only a few hours per session, dissociation cannot catch up, and only the tiny free fraction crosses. A 2014 review in Toxins listed these toxins among the reasons cardiovascular complications stay high in dialysis patients — a claim still being debated.
So even on the narrow question of "removing waste," a single dialysis session takes away the easiest molecules — urea, creatinine — and leaves the hardest ones right where they were.
More Dialysis, Faster Kidney Decline
Clearance efficiency is not even the main problem. The more dialysis a patient gets, the faster their remaining kidney function disappears.
The Frequent Hemodialysis Network's nocturnal trial randomized patients to dialysis 6 times or 3 times per week. At baseline, 63 participants still had residual kidney function, with mean urine volume 0.76 L/day and urea clearance 2.3 mL/min. The 2013 report in Kidney International followed them: at 12 months, urine output had dropped to zero in 67% of the 6-times group versus 36% of the 3-times group. The gap appeared early — at month 4 it was already 52% versus 18%.
| Follow-up point | Dialysis 6x/week — urine output zero | Dialysis 3x/week — urine output zero |
|---|---|---|
| Month 4 | 52% | 18% |
| Month 12 | 67% | 36% |
| Baseline: 63 participants still had residual kidney function (mean urine volume 0.76 L/day, urea clearance 2.3 mL/min). Data: Daugirdas JT, et al. Kidney Int 2013;83:949-958 (FHN Nocturnal Trial). | ||
The daily trial from the same network, published in the New England Journal of Medicine in 2010, found the 6-times group needed more vascular-access interventions, with a hazard ratio of 1.71 (95% CI 1.08–2.73). Extra punctures, clots and repairs all land on the same arteriovenous fistula. Even in sick patients, more dialysis becomes too much.
Why residual function falls so fast is not fully understood. The leading hypotheses: dialysis repeatedly swings blood volume and osmotic pressure, so kidney perfusion fluctuates — one 2025 ultrasound study measured kidney perfusion dropping about 17.5% during a single session; or solute concentrations are pressed so low that tubules lose their working load. Some nephrologists borrow the ventilator-induced-lung-injury analogy and ask whether dialysis itself can injure a kidney, in the same way a breathing machine can injure a lung. And reviews of incremental dialysis make a parallel point: an abrupt, full-intensity start may itself accelerate the loss of the kidney's remaining function.
Starting Earlier Doesn't Help Either
What about people with kidney disease who start dialysis earlier — surely that helps?
