#18 2004 · New England Program for Kidney Exchange · Organ transplantation
Economists chained incompatible donors until every kidney found its match
the problem
Willing donors, wrong blood types
background
In 2003 roughly 60,000 Americans were waiting for a kidney; only 8,665 deceased-donor transplants happened that year, and 3,436 people died waiting. Live donation could fill some of the gap — a healthy person has two kidneys and can live with one — except that a willing donor is frequently the wrong blood type or tissue match for the specific patient they love, and money cannot legally fix that: buying or selling organs is illegal everywhere transplant medicine is regulated.
A handful of hospitals had begun swapping kidneys between two incompatible pairs — my incompatible donor gives to your patient, yours gives to mine — and Johns Hopkins had even managed a few three-way versions. But as of December 2004 only five such exchanges had happened across all fourteen transplant centers in New England, because no one had built a database of incompatible pairs to match against; unmatched donors were simply sent home.
what everyone would do
Grow the deceased-donor pool through more organ donation campaigns, or wait for medical advances that loosen compatibility requirements — the two levers hospitals and regulators actually had, since money can't legally be used to clear the mismatch between a willing donor and a patient who needs a different blood type.
what they saw
A donor who's the wrong match for the patient they love isn't a wasted resource — they're the right match for someone else's patient, and vice versa. The shortage wasn't a shortage of kidneys, it was a shortage of a system that could find and connect the pairs whose mismatches happened to cancel each other out.
the move
Economists applied matching theory to incompatible donor-patient pairs, later extended into long non-simultaneous chains.
why it works
Treating each incompatible donor-patient pair as a node in a matching market, rather than a failed individual case, turns what looks like scarcity into an allocation problem economists already knew how to solve: build a registry of every incompatible pair, then search it for cycles and chains where each donor is compatible with someone else's patient. A two-way swap needs both surgeries simultaneous (neither donor can back out once their patient has received a kidney), but non-simultaneous chains kicked off by an altruistic, unmatched donor remove that constraint and let the match run arbitrarily long, so the clearinghouse's total transplants grow with the size of the pool searched, not with any new kidney supply.
the payoff
Thousands of transplants that could not otherwise happen; the 2012 economics Nobel cited the work.
where it breaks
It only works where a formal registry can be built and legally operated — the mechanism depends on centralizing information about incompatible pairs that would otherwise never be compared, and on a legal environment (like US transplant law) that permits organized non-monetary exchange while still banning payment. It also needs enough participating pairs to make matches likely; a clearinghouse with too few incompatible pairs in the pool, or a rare blood type with no reciprocal match anywhere in it, still leaves some donors and patients unmatched no matter how the algorithm searches.
what came after
In September 2004 the Renal Transplant Oversight Committee of New England approved a formal kidney-exchange clearinghouse, proposed jointly by transplant physicians Francis Delmonico and Susan Saidman and economists Alvin Roth, Tayfun Sönmez and Utku Ünver — turning an ad hoc, five-exchange-a-year practice into a designed market. The matching mechanism the economists built later extended beyond simple pairs into long donation chains kicked off by altruistic donors, and Roth shared the 2012 Nobel Memorial Prize in Economic Sciences for this and related market-design work.
references
- [1]Kidney ExchangeNational Bureau of Economic Research (Roth, Sönmez & Ünver), 2003nber.org
- [2]Roth shares economics NobelHarvard Gazette, 2012news.harvard.edu