A donor dies in a hospital in one city, and within hours a computer in Virginia has to decide which of more than a hundred thousand strangers receives the organ. How are organs allocated for transplant in that narrow window is one of the most consequential automated decisions in American medicine — and it is nothing like the orderly queue most people picture.
Here is the short version. When an organ becomes available, software run under federal contract builds a fresh, ranked list of candidates for that one organ, scoring each on medical urgency, tissue compatibility, how much they stand to benefit, and how far the organ has to travel. Time on the list matters, but it is rarely the thing that decides.
Who runs the list, and under what law
There is only one such list, and it is national. The Organ Procurement and Transplantation Network, created by the National Organ Transplant Act of 1984, is the single system every American transplant centre plugs into. For nearly four decades it was operated under federal contract by the United Network for Organ Sharing, a non-profit based in Richmond, Virginia.
The moment an organ is recovered, the network runs what it calls a match. The software takes the donor’s characteristics and generates a ranked list of every candidate who could safely accept that specific organ. Before ranking begins, anyone the organ physically cannot go to is screened out:
- an incompatible blood type
- a body size the organ does not fit
- antibodies primed to attack the donor tissue
- in some cases, a candidate too far away for the organ to reach in time
What survives that filter is then scored — and the scoring is where fairness and arithmetic collide.
Why it isn’t first come, first served
The reason is scarcity. More than 100,000 people are on the national waiting list, according to the federal government’s organ donation figures, and roughly 17 of them die each day before an organ arrives. A new name is added about every eight minutes. There are nowhere near enough donors to clear the backlog, so every organ has to do the most good it can.
A pure queue would fail at both ends. It would let the sickest patients die while healthier people ahead of them waited their turn, and it would drop a scarce organ into a poor match that fails within a year. So waiting time is only one input among several, and for some organs it barely moves the ranking.
This is the same tension that runs through any system rationing something there is not enough of — the logic that decides who gets bumped when a flight is overbooked is a cruder cousin of it. The difference here is that the stakes are measured in lives, and the rules are written to reflect that.
Two caveats before the mechanics. A minority of transplants skip the list entirely, when a living person gives a kidney or part of a liver to someone directly. And most people who die never become donors at all — usable organs generally come only from the small number who die on a ventilator with circulation maintained, which is why the shortage is so stubborn.
Every organ has its own formula
There is no single transplant algorithm. Each organ runs on its own scoring system, tuned to how that organ fails and what a good outcome looks like. A liver and a kidney are ranked by almost entirely different rules.
The table below shows what each system is built to reward.
| Organ | What the priority score is called | What it mainly rewards |
|---|---|---|
| Liver | MELD 3.0 (PELD for children) | How likely the patient is to die soon without a transplant |
| Kidney | KDPI and EPTS points | Matching long-lasting kidneys to patients likely to live longest |
| Heart | A six-tier status system | Immediate medical urgency, the top tier offered first |
| Lung | Composite Allocation Score | Urgency, expected survival, biology and distance combined |
Those differences are not arbitrary. A failing kidney can be managed for years on dialysis, so the system can afford to optimise for the best long-term match. A failing liver or heart has no machine that can stand in for long, so urgency takes over. The formula follows the biology.
How a liver’s priority is decided
Livers are ranked mostly by how close the patient is to dying. The tool is the Model for End-Stage Liver Disease, or MELD — a score built from blood tests that predict short-term mortality. The higher the number, the sicker the patient, and the higher up the list they climb.
Since 2023 the network has used an updated version, MELD 3.0. It draws on bilirubin, creatinine, the clotting measure known as INR and blood sodium. The newest version added serum albumin and a small adjustment — 1.33 points — for women, after data showed female candidates were being under-prioritised, and it capped the creatinine value at 3.0 so kidney trouble could not swamp the score.
None of that is a bedside judgement. The score is a statistical bet on the near future, drawn from thousands of past cases — the same kind of outcome data that decides whether a new drug is judged to work. It is coldly numerical on purpose, so that two patients with the same blood chemistry rank the same wherever they are treated.
There are exceptions. A patient with liver cancer that has not yet spread can be granted extra points, because the raw blood tests would not capture how urgent the case really is. Those exception scores are reviewed by a national board rather than handed out locally.
Why kidneys reward the best long-term match
Kidney allocation, overhauled in 2014, is built around a different goal: not wasting good kidneys. Two numbers do most of the work. The Kidney Donor Profile Index rates each donated kidney from 0 to 100 per cent, where a lower figure means a longer-lasting organ. The Estimated Post-Transplant Survival score does the same for candidates.
The system then tries to line them up. The kidneys expected to last longest — those with an index of 20 per cent or better — are steered towards the roughly one in five candidates expected to live longest after surgery.
The idea, called longevity matching, is to avoid putting a decades-good kidney into a patient who will not outlive it, or wasting a short-lived organ on someone who could have used a better one. It is efficiency written into policy, and it is the part critics find coldest.
Two other factors weigh heavily. Patients whose immune systems are highly sensitised — measured by a figure called CPRA — are very hard to match, so the system bumps them up the list whenever a rare compatible organ appears.
Waiting time, meanwhile, now counts from the day a patient started dialysis, not the later day they were formally listed. That change corrected an old bias against people who were referred to a transplant centre late — often the poor and the uninsured, who reach specialists slowest.
Hearts and lungs race the clock
Thoracic organs are ranked on urgency above almost everything else, because they cannot wait. A heart can be kept outside a body for only a handful of hours.
Heart candidates are sorted into a six-tier status system, brought in during 2018 to draw finer distinctions than the three tiers before it. The top status is reserved for patients on the most aggressive mechanical support — the machines that keep a failing heart pumping — whose survival is counted in days. They are offered organs first, within the distance a heart can safely travel.
Lungs moved in 2023 to a Composite Allocation Score, the first organ to adopt the newer model the whole system is drifting towards. Rather than rank by urgency and then break ties, it folds urgency, expected survival, the candidate’s biology, paediatric status and distance into a single weighted number for every donor-and-candidate pairing.
The slow end of geography
For most of the system’s history, geography was a hard wall. The country was carved into donation service areas, and an organ was offered to everyone inside the local area before anyone outside it — even a far sicker patient a short drive across the boundary.
Courts and critics argued that an invisible line should not decide who lives. In 2018 the network’s board committed to phasing the boundaries out in favour of what it calls continuous distribution: instead of a wall, distance becomes just one more factor in the score, its weight sliding down smoothly as the organ has to travel further.
Lungs went first, in 2023. Kidneys, the pancreas and the rest are being redesigned the same way, one organ at a time. It is a slow rebuild, and each organ’s committee has to settle how much weight distance should carry against urgency — a genuinely hard trade-off with no obviously right answer.
Can money or fame move you up the list?
Not on the list itself. The matching software has no field for income, celebrity, race or religion; it cannot see them. A billionaire and an uninsured labourer with identical scores rank identically for the same organ.
Where money talks is earlier, in getting listed at all. A patient wealthy enough to travel can register at more than one transplant centre in different regions, improving the odds that an organ turns up nearby while they wait.
It is a legal tactic, and one that plainly favours anyone who can afford the airfare and a second round of evaluations. Getting referred, assessed and listed in the first place depends just as much on having insurance and on living within reach of a transplant programme at all.
Where it still breaks down — and what to watch
The algorithm is only as good as the organs that reach it, and too many never do. Usable organs are still discarded, sometimes because no centre accepts them in time, sometimes because of the plain logistics of moving them fast enough. The share of recovered kidneys that end up thrown away has troubled researchers for years.
Oversight has been the weaker half of the system. A 2020 Senate investigation found that basic errors — including failures to complete mandatory blood-type checks — were far more common than the public knew, and that between 2007 and 2015 around 70 people died after receiving organs that transmitted a disease. Complaints to the network, it concluded, rarely changed much.
That pressure produced change. In September 2023 Congress passed a law ending a monopoly that had stood for four decades, letting the government open the network’s work to other contractors for the first time.
The move was reported alongside a congressional investigation into how the old system had performed. That reporting noted the roughly 17 daily deaths fall disproportionately on patients of colour and rural Americans — evidence that a colour-blind algorithm does not guarantee colour-blind outcomes.
So watch two things as continuous distribution spreads to more organs. Does the share of usable organs that get discarded fall, and does the gap between who gets listed and who gets transplanted narrow? The formula can be made fairer line by line. Whether the system around it delivers is the harder question.
