Tests 90% accurate

The tests have been shown to be about 90% accurate at detecting the presence of amyloid plaques, helping doctors determine whether a patient’s cognitive impairment is being caused by Alzheimer’s or if they should consider a different disease.

“With these blood tests, we really think that there is a possibility of providing many more people with a diagnosis, which is really exciting,” said Dr Suzanne Schindler, an associate professor of neurology, WashU Medicine in St Louis.

The accuracy of the tests depends on how much modified tau is in the blood. If the levels are very low, experts feel fairly confident a person does not currently have amyloid deposits in the brain, and if the levels are high, then there is a very strong chance the patient does.

But people with levels in the middle are more ambiguous. In those cases, experts often recommend getting a PET scan or cerebrospinal fluid test to confirm an Alzheimer’s diagnosis.

What if you have no symptoms but are worried about getting Alzheimer’s someday? There’s a lot of excitement about using the tests to screen people who might develop Alzheimer’s symptoms in the future, and some direct-to-consumer lab test startups, like Function, already offer them. But the experts interviewed for this article tended to recommend against that, for now.

For one, the tests are less accurate at detecting amyloid plaques in people without cognitive impairment. “If someone has a negative test, that person very likely does not have signs of Alzheimer’s in the brain,” said Dr Christopher Rowe, director, Australian Dementia Network, University of Melbourne. But if the test comes back positive, he estimated, it’s a 50/50 chance whether the person has plaques or not.

“I’m being a little pessimistic perhaps, but the message is if you do have a positive result, there’s a fair chance it’s a false positive,” he said. “Even if amyloid plaques are present, it doesn’t mean they’ll lead to cognitive impairment. In some ways, plaques in the brain can be thought of like plaques in the arteries.”

Just like having atherosclerosis doesn’t guarantee that a person will have a heart attack, having amyloid build-up doesn’t guarantee that someone will develop dementia.

According to one recent study from the Mayo Clinic, depending on how much amyloid plaque someone had in the brain, the chances of developing mild cognitive impairment or dementia ranged from 56% to 84%. “If you’re positive on these tests, that does not necessarily mean that you will develop symptoms of Alzheimer’s,” Schindler said. “But it definitely means you’re at higher risk.”

And unfortunately, if someone does have amyloid plaques in the brain, there isn’t much that can be done about it right now. A couple of studies have found that physical activity and other lifestyle interventions may help lower tau blood levels or delay tau tangles from developing.

But until the results from the clinical trials aiming to delay dementia come out, that’s the extent of the current options.