In 2018, Beth’s mother died of Alzheimer’s disease at 75. “It was terrible,” she said. And once you’ve gone through that, she added, “you start to become concerned about your own future.”

A year ago, Beth, now 58, learned about a new blood test for Alzheimer’s. Her primary care doctor agreed to order the test in June, given her family history. It found that levels of a protein called p-tau217 were elevated, suggesting she had early signs of Alzheimer’s in her brain.

“I would love to say that it was a complete shock, but it wasn’t,” Beth said. “I don’t want to sound all doom and gloom. I just — I feel like it’s coming for me.” (To protect the subjects’ privacy when discussing sensitive health information, The Times is identifying them by first name only.)

A neurologist ran several follow-up tests and found that Beth’s cognition was normal. The doctor also told her that the blood tests aren’t 100 percent definitive. “That makes you feel better,” Beth said. “But then it throws a lot more question marks into the whole situation, too.”

Questions like: What does the future hold? Will she be a burden to her family? How long can she keep working? If she doesn’t travel to Paris next year, will she ever be able to go?

Two blood biomarker tests for Alzheimer’s were approved by the Food and Drug Administration last year to help diagnose people who are experiencing cognitive impairment.

More and more, people without memory problems are taking the tests, as well, to try to determine whether they will develop Alzheimer’s in the future. Some purchase them through direct-to-consumer lab testing sites, others are referred by a doctor. However, dementia experts almost universally say the tests aren’t ready to be used in this way and recommend against doing so.

For some patients, the results are devastating, leaving them feeling scared and helpless. Others view them as empowering and have jumped into action to try to improve their overall health and get their affairs in order. And many, like Beth, are confused about what their results actually mean.

Open Questions

The tests measure blood levels of two proteins, amyloid and tau, that become dysfunctional in the brains of people with Alzheimer’s. One test in particular, called p-tau217 for the form of the protein it measures, correlates especially well with amyloid, which can accumulate decades before someone starts to experience cognitive impairment. P-tau217 is quickly becoming the preferred predictive test: A study published last month in the journal JAMA found that cognitively normal people with very high levels of p-tau217 had a 38 percent chance of developing memory and thinking problems within five years, and a 78 percent chance in 10 years.

Dr. Jeff Williamson, the chief of gerontology and geriatric medicine at Wake Forest School of Medicine, said that in the past 18 months he’s seen two to three dozen cognitively normal patients, most in their 50s and 60s, who’ve taken the test. Many have parents or other family members with Alzheimer’s, and the results have filled them with worry about what may be to come.

“Every time they forget something, or they misplace their keys or forget a classmate’s name, they actually start developing an anxiety that’s sort of, ‘Ah, this may be the Alzheimer’s that’s in my blood,’ even though their memory test was normal last month or four months ago,” he said.

While the odds are certainly greater that someone with a high p-tau217 level will someday have dementia, a diagnosis isn’t guaranteed. A couple experts said the connection between p-tau217 and Alzheimer’s is akin to how high cholesterol increases the risk of a heart attack.

There can be accuracy issues with the tests, as well, particularly when people have moderate levels of p-tau217. In those cases, the test isn’t as reliable as when people have very low or very high levels, said Rachel F. Buckley, an associate professor of neurology at Mass General Brigham and Harvard Medical School, who led the recent JAMA study. “I would become far less confident” in the results in that situation, she said.

Other factors can also affect accuracy. Doctors often confirm the results with follow-up testing, and when doing so, they sometimes catch false positives.

Dr. Richard Isaacson, a preventive neurologist at the Atria Health Institute in Florida who specializes in caring for people at risk of dementia, said he sees false positives “every week.” He gave one example of a woman whose test was taken before the 4th of July holiday weekend and analyzed after. He thinks the dry ice the sample was packed in melted during that time, altering the sample and producing a bad result. In another instance, a patient had a false positive reading because he had rhabdomyolysis, where muscle tissue breaks down and releases proteins into the blood stream. Kidney disease can also skew blood test results. (Dr. Isaacson also develops brain biomarker tests at AlzLabs in Florida.)

There are potential legal ramifications to consider, too. Biomarker tests don’t have privacy protections, said Jalayne Arias, an associate professor in health policy and behavioral sciences at the Georgia State University School of Public Health. That leaves people vulnerable to discrimination from life insurers, long-term care insurers, retirement or assisted living communities or even from employers.

“There’s this tension because certainly this information could provide somebody with knowledge that would help them make plans, make decisions about where they want to live, how they want to spend their time,” said Ms. Arias, who recently wrote about these concerns in the journal Neurology. “Yet the documentation of this in their medical record or disclosing this to a third party may inadvertently curtail the options that are in front of them.”

Future Hope

Experts hope that these tests will become more reliable at predicting who’s at risk of Alzheimer’s — and even more important, that there will be a drug available to delay or prevent the onset of symptoms. If that happens, most experts said they would start recommending cognitively healthy people get tested.

But as of now, an effective preventive therapy doesn’t exist. Two drugs that are used to treat Alzheimer’s are being tested for their protective potential in clinical trials, and results are expected in a year or two.

In the meantime, Dr. Williamson and Dr. Isaacson both advise patients with positive tests to adopt a healthy lifestyle and focus on improving other aspects of their health, like getting blood pressure under control.

A study published last year found that, regardless of people’s p-tau217 levels, participants who were healthier (without many major chronic conditions and who regularly exercised, ate a nutritious diet and didn’t smoke) had a lower risk of developing cognitive impairment over the next eight years than those who were less healthy.

These behaviors don’t appear to affect the buildup of amyloid or tau in the brain, but they do offer a sort of “buffer” against the harm that those proteins can cause, said Sterling Johnson, a professor of geriatrics and dementia at the University of Wisconsin-Madison School of Medicine and Public Health, who led the study.

That’s the strategy Susan, 70, is taking. When she took a p-tau217 test in June and found out that her levels were elevated, she “was kind of gobsmacked,” she said. Even though her father died from advanced Alzheimer’s, she wasn’t experiencing any problems with her memory.

In the weeks since Susan got her results, she’s made a concerted effort to exercise more, eat a Mediterranean-style diet and do brain puzzles instead of scrolling on social media. She also confirmed that her will and medical power of attorney were up-to-date.

Susan knows the blood test doesn’t guarantee she’s going to develop Alzheimer’s. But “it gives you a kick in the butt to get busy doing things you should be doing anyway,” she said.