Almost half of elderly people don't have an accurate sense of how much longer they're likely to live, a problem that may lead some of them to make poorly informed medical decisions, a U.S. study suggests.

Doctors and patients often consider life expectancy when deciding which preventive measures or treatments will do the most good.

This projection might, for example, lead a cancer patient to skip toxic chemotherapy if they're not likely to live long enough to benefit from it, or it might encourage someone with diabetes to make lifestyle changes that might improve the last few decades of life.

"Whether they are aware of it or not, patients have a sense of how long they have to live and (they) bring this sense of prognosis to their healthcare decisions," lead study author Rafael Romo of the University of California, San Francisco, said by email.

Romo and colleagues asked more than 2,000 adults age 64 and older to predict their odds of living at least another 10 years and compared their responses to objective calculations doctors might use as the basis for clinical recommendations.

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Overall, about 55 percent of participants had estimates similar to the objective calculations, the researchers report in JAMA Internal Medicine.

But 33 percent of them underestimated how long they might live and 12 percent overestimated their probable longevity.

Unrealistic expectations may make patients less likely to follow recommended treatments or change long-standing regimens, the authors argue.

"The logic goes that if patients underestimate their survival they may not think it is worthwhile to stop smoking, or eat healthy meals or stay active or undergo cancer screening tests or other preventive care," Dr. Victor Montori, an expert in shared decision making at the Mayo Clinic in Rochester, Minnesota, said by email.

"Conversely, if patients overestimate their survival, they may demand healthcare from which they cannot derive benefit as they will run out of time," added Montori, who wasn't involved in the study.

Some patients might choose a palliative rather than a curative approach to end-of-life care, if they realized that whatever time they had left would be spent enduring the side effects of treatments they won't live long enough to benefit from, noted Dr. Rosanne Leipzig, a geriatrics specialist at the Icahn School of Medicine at Mount Sinai in New York.

Others with decades to live might refuse medications to control their blood pressure or manage diabetes because they didn't expect to live very long and wanted to avoid a daily drug regimen that will benefit them years down the line, Leipzig, who wasn't involved in the study, said by email.

When doctors and patients don't discuss life expectancy, physicians are likely to err on the side of offering more treatment rather than less, said Dr. Elizabeth Eckstrom, a geriatrics researcher at Oregon Health and Sciences University in Portland who wasn't involved in the study.

The study findings suggest that patients may often be surprised to learn they have longer to live than they anticipated, Eckstrom added by email.

"Patients want to know prognosis to prepare logistically and financially, to prepare psychologically or spiritually, to prepare friends and family, to make the most of the time they have left, and to make health-related decisions," Eckstrom said.