The results illustrate the value of people making their wishes known in a living will and designating someone to make treatment decisions for them, the researchers said.
In the study, those who spelled out their preferences in living wills usually got the treatment they wanted. Only a few wanted heroic measures to prolong their lives.
The researchers said it's the first accounting of how many of Medicare to pay doctors for counseling patients about end-of-life issues like living wills.
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Critics labeled the counseling "death panels" and the proposal was eventually dropped before the researchers could get their report out. They had pushed to complete the study because of the national debate, but it took time to get it published, said the study's leader, Dr. Maria Silveira. She teaches at the University of Michigan and does research for Veterans Affairs.
The study is in Thursday's New England Journal of Medicine.
The researchers concluded that advance directives - living wills and health proxies chosen to make end-of-life decisions - are "important tools for providing care in keeping with patients' wishes."
The use of these directives has increased in the U.S. despite debate about their effectiveness. For the past two decades, hospitals and facilities that take Medicare patients are required to provide information on them.
A living will states a person's choices for treatment if he becomes incapacitated, but critics complain they are too vague to be helpful. A health care proxy names another person - usually a relative or friend - to make medical decisions if needed. Many people have both.
Typical decisions involve the use of breathing machines or feeding tubes or giving someone CPR.
In the study, researchers looked at how often the elderly reach the point where they can't make their own care decisions near the end of life - usually because of dementia, a stroke or a debilitating illness. They also examined how many had living wills or a proxy and the outcome.
The study included 3,746 people age 60 and older who died between 2000 and 2006. The average age was 80.
About 30 percent needed a treatment decision made before death but couldn't do it themselves. Of those, about two-thirds had either a living will, a proxy or both.
After the person died, relatives were interviewed to find out if the person's wishes were followed. Most reported that they had. Nearly all the patients had wanted limited or "comfort" care; only 2 percent wanted aggressive care.
Advance directives are available for individual states online, http://www.caringinfo.org/PlanningAhead.htm, and an attorney isn't needed, a popular misconception, Silveira said.
"We don't expect perfection out of these documents," she said. "They're there to make a difficult situation maybe a little bit less so."
The study's results, while "tantalizing", haven't convinced Dr. Muriel Gillick of Harvard Medical School that living wills are all that useful. Ideally, older patients, along with their proxy, should discuss their medical condition, goals and treatment options with a physician - instead of just signing a form, she said.
In an editorial in the journal, Gillick said the findings nevertheless "demonstrate that talking about the goals of medical care has become acceptable to a large majority of Americans who need it most."
An alternative, she said, is a program with a more detailed form that includes doctor's orders for specific care - called "physician orders for life-sustaining treatment." The program has been adopted in a few states.
One community that has embraced advanced directives is La Crosse, Wis. A citywide program grew out of the counseling experience of Bernard "Bud" Hammes at Gundersen Lutheran hospital. Hammes said he saw how distressing it was for three families who had to decide whether to continue dialysis for patients who had suffered serious strokes.
He asked them what their relative would want. "In all three cases, the family said: 'We have no idea. We never talked about it,"' he recalled.
A program called Respecting Choices is now used by the city's hospitals, nursing homes and other health-care providers. Today, most patients - 85 percent - have a care plan when they die, he said.