The new system replaces one that relies heavily on how long a patient has been on a waiting list, although it does not break down the geographic boundaries that allow much shorter waits in some parts of the country.
The new system gives each patient a score based on three lab tests and is expected to predict better which patients will die without transplants.
"It's a much more objective way to rank patients," said Dr. Richard Freeman, a liver surgeon at Tufts University, who chairs the liver committee for the United Network for Organ Sharing.
The transplant network, meeting in Alexandria, Va., voted 33-0 to implement the new system, although it cannot take effect until the network's computers are reprogrammed, probably in February.
Development of the system began amid a fight over the fairness of geographic boundaries that govern the distribution of organs.
Because patients and donors are not distributed evenly around the country, the wait for a liver is much longer in some places than it is in others. Critics say it's wrong for a patient in one city to die while a healthier patient elsewhere gets a transplant.
That fight has cooled, and advocates of the new liver distribution method hope critics will appreciate the effort to get livers to the sickest patients, at least within each area.
"This is science now," Freeman said. "We're looking at this in scientific ways rather than emotional ways, which is really good for patients."
Still, the geographic boundaries remain. Patients will be able to compare the score needed to get a liver in one part of the country vs. another.
Under the current system for distributing livers, patients are grouped into four broad categories. Within each group, those waiting the longest get first chance at the livers.
But many transplant experts believe waiting time is a poor way to measure how sick a patient is. For instance, some doctors encourage patients to get on the list before they really need a transplant so they can accumulate waiting time and be closer to the top when they need it.
The new policy retains just one of the current categories. Patients classified as Status 1, who have become suddenly ill and are expected to die within a week, still will get first chance at donated livers.
Everyone else will get a number. For adults, they range from 6 to 40, with the highest numbers for the sickest patients. For children, they range from minus-10 to 70.
Patients who got transplants under the current system had an average score of 18.
When a similar system was implemented in New England, there was a 31 percent reduction in the number of people who died on the waiting list, Freeman said.
Patient scores will be determined by a combination of three medical factors: the body's ability to clot blood; the ability to beak down hemoglobin; and kidney function, which can be affected by a failing liver. The system was first developed by the Mayo Clinic in Rochester, Minn., and is known as MELD, for Model for End-Stage Liver Disease.
Ties in patient scores will be broken by waiting time.
The network also agreed to restrict further who can get livers from donors with blood type O. Medically, these livers can go to any patient, disadvantaging those with type O, who can receive only O livers.
Under the new policy, only patients with blood type O, or with type B who have a score above 20, can get O livers.
The network will now submit the policies to the Department of Health and Human Services. Under a new regulation giving HHS more oversight authority, the department may order changes.
The nation faces an acute shortage of livers, with about 18,500 people waiting for donated livers. Some 5,000 liver transplants were performed last year, but 1,636 people died waiting.
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