However, researchers conclude, the flip side of this "opt-out" approach may be a lower rate of transplants from living donors. A number of countries, mainly in Europe and South America, have presumed-consent organ-donation systems.
This means that people have to opt out of the system if they do not wish to have their organs harvested after they die; surviving family members are still consulted and can object if they want.
In contrast, countries such as the U.S., Canada, Australia and the U.K. have explicit-consent systems, where people have to designate themselves as organ donors; in cases where a deceased person's wishes are unknown, the family can grant permission for organ donation.
Advocates of presumed consent say it helps address donor-organ shortages.
And several studies have found that nations with presumed consent tend to have higher rates of deceased-donor organ donation than those with explicit consent.
Those studies did not, however, look at effects on organ transplants from living donors, the vast majority of which are kidney transplants. (Living donors can also give a segment of the liver or, in a small number of cases, a portion of lung, pancreatic or intestinal tissue.)
For the new study, reported in the Annals of Internal Medicine, researchers examined rates of living- and deceased-donor kidney transplants in 44 countries between 1997 and 2007.
Half of the countries had presumed-consent systems, including France, Germany, Italy and Spain; the other half, including the U.S., Canada, Japan and Australia, had explicit consent.
In general, presumed-consent nations had a higher rate of deceased-donor transplants than explicit-consent nations did -- with a median, or midpoint, rate of 22.6 transplants per million people in the population, versus 13.9 per million in countries with explicit consent.
When it came to living donations, however, presumed-consent nations had a transplant rate of 2.4 per one million people, compared with 5.9 per million in explicit-consent countries.
"We were surprised to see such low rates of living kidney donation in nations with presumed consent," senior researcher Dr. Amit X. Garg, of the London Health Sciences Center in Ontario, Canada, told Reuters Health in an e-mail.
The findings do not prove that a presumed-consent system itself discourages kidney donations from living donors, according to Garg and his colleagues. But one possibility, they speculate, is that in countries with the policy, "the public perceives that the need for kidneys is addressed by presumed consent legislation."
Another potential explanation is that in countries with presumed consent, there is more reluctance to have healthy living donors face the risks of surgery. "I think this shows there is a nervousness, ethically, about using living donors," Dr. Arthur L. Caplan, director of the Center for Bioethics at the University of Pennsylvania in Philadelphia, told Reuters Health.
The findings do not mean that presumed consent is an ineffective way to address donor-organ shortages, according to Caplan, who was not involved in the study and supports presumed-consent donation -- or what he terms "default to donation." He noted that whole organs other than the kidneys -- including the heart, lungs, pancreas and intestines -- must come from deceased donors, so boosting the availability of cadaver organs across the board is vital.
There are nevertheless reasons to be concerned about the potential for driving down living donations for kidney transplants: those done with living donors tend to be more successful than transplants using cadaver kidneys.
One advantage, Caplan said, is that the operations are planned, whereas those done when a deceased-donor organ becomes available are more akin to emergency surgery. Kidneys from living donors also tend to last longer.
Based on U.S. data for 2000 to 2006, 80 percent of all kidneys transplanted from living donors were still functioning five years later. That compares with 67 percent of those from deceased donors.
But transplants from living donors also mean putting a healthy person at some risk, for example, of bleeding and infection from surgery. Over the long term, donors also need to have regular blood-pressure checks and blood and urine tests to evaluate their kidney function; research suggests, though, that they are not at increased risk of kidney failure. Garg said that the decision over whether to adopt a presumed-consent system involves many considerations.
"We looked at one particular issue: What are rates of living kidney donation in nations with presumed consent compared to those with explicit consent?" However, he also asserted that nations considering presumed consent would have to educate both health providers and the public that the policy, by itself, "will not be sufficient to meet the demands for organs." "In other words," Garg said, "such countries should continue to support efforts and infrastructure to safely expand the practice of living kidney donation."
Caplan said that in countries moving toward presumed consent, doctors could still talk to kidney patients and their families about the option of living donation -- which could help reduce any negative effects on living-donation rates.
But he also said, "I think that if we had all the cadaver organs we needed, we wouldn't do living donations at all." In the U.S., about 87,000 people are currently on the waiting list for a donor kidney, accounting for more than three-quarters of all Americans waiting for an organ.
Based on 2007 figures, the average wait time for a kidney is more than three years.
But Caplan said there is no strong movement in the U.S. toward adopting a presumed-consent system as a way to address organ shortages -- although some individual states may eventually do so.
In 2008, Delaware lawmakers introduced legislation to create a presumed-consent program, but it stalled; earlier this year, a similar bill was introduced in New York by a state assemblyman whose daughter received a kidney from a deceased donor.
Presumed consent may be a tough sell in the U.S., according to Caplan.
"The term 'presumed consent' kind of gets stuck in people's throats," he said. "We don't like to be told someone is 'presuming' our consent for anything."
Simply changing the law would not be enough, Caplan said.
Both health providers and the public would have to be educated about how a presumed-consent system would work.
SOURCE: Annals of Internal Medicine, November 16, 2010.