The findings aren't enough evidence for doctors to urge more people to get genetic testing, said lead author, Dr. Robert Green. But they challenge assumptions that people will be devastated by a positive test result and misread it as certain proof they're doomed to Alzheimer's.
"Our participants were able to understand the risk and manage it," said Green, a researcher at the Boston University School of Medicine.
For many who learned they were not at increased risk, testing wound up being a great relief.
The study is being published Thursday in the New England Journal of Medicine.
Alzheimer's is the most common cause of dementia among older adults, and affects as many as 5 million Americans, most of them over age 60.
The exact cause is not known, but scientists believe heredity plays a role. People with a certain gene - known as APoE4 - are believed to be three to 15 times more likely to develop the illness than other people, depending on how many copies of the gene they have. It is not, however, a sure sign someone will get the disease.
Experts recommend a test for APoE4 only to diagnose Alzheimer's in patients already having symptoms. But a number of companies have been marketing the test directly to consumers, even though there is no cure for the disease or way to prevent it from developing.
"It's available if somebody wants it," said Beth Peshkin, a genetic counselor at Georgetown University Medical Center.
The study measured anxiety, depression and stress levels in 162 healthy adults who were children or siblings of people with Alzheimer's. The participants were in their early 50's, on average. Most wanted to know if they had the gene, and people who already had severe anxiety or depression were not included.
All got a blood test that looks for the gene. Two-thirds were given the results, the other third were not.
Overall, the anxiety, depression and distress scores of those who got the results and those who didn't were about the same.
About half of those given results tested positive for the gene. Predictably, those who got the good news that they tested negative had slightly lower anxiety, depression and distress scores than anyone else. But those who tested positive were as likely to say they would still get the test if they had the chance to do it over.
The results are not so different from some studies involving genetic testing for other conditions, like Huntington's disease or some types of cancer, said Elizabeth Thomson, an official at the National Human Genome Research Institute.
"It seems to be worse on people not to know than to know that they are positive or negative," said Thomson, director of the federal agency's ethical, legal and social implications research program.
Indeed, in the Alzheimer's study, 16 percent of those in the group that did not get results dropped out - a higher proportion than those who left the other group. That may have been a sign of frustration of people who really wanted the information, said Green.
"Once you make up your mind that this is something you're interested in learning, you really don't want to be told no," Green said. It's possible that stress levels were elevated in the people who didn't get the results, he said.
The study was funded by the National Institutes of Health. Two of the authors - not Green - last year received consulting fees from Smart Genetics, a Philadelphia company that provided direct-to-consumer genetic testing. The company has since gone out of business.
Testing remains a complicated decision, even for people who know a lot about it. Peshkin, the genetic counselor, said her 80-year-old father has Alzheimer's, but she has decided not to get the test.
"Even though I understand the uncertainty of how predictive the test is, it would be something I'd be very anxious about," said Peshkin, 41. "I would be one of those people that the first moment I couldn't remember someone's name, I'd think 'Oh God, here it comes.' I'd rather not know."