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New blood tests can help diagnose Alzheimer's — but some aren't as accurate as others

MARY LOUISE KELLY, HOST:

A new generation of blood tests is poised to change the way doctors diagnose Alzheimer's disease. The best of these tests can accurately detect the brain changes associated with Alzheimer's without the need for a brain scan or spinal tap. But NPR's Jon Hamilton reports some tests are more accurate than others.

JON HAMILTON, BYLINE: The hallmarks of Alzheimer's are sticky amyloid plaques and tangled fibers that build up in the brain. Dr. Suzanne Schindler is a dementia specialist at Washington University School of Medicine in St. Louis. She says detecting those changes is tricky in a living patient.

SUZANNE SCHINDLER: It used to be that the only way that you could definitively diagnose someone with Alzheimer disease is by doing an autopsy.

HAMILTON: In the past couple of decades, scientists have found ways to detect the presence of plaques and tangles using PET scans or tests of spinal fluid. But the scans are costly and spinal taps are unpopular with many doctors and patients. So Schindler and her colleagues got a lot of attention a few years ago when they showed that amyloid plaques could be revealed by a blood test.

SCHINDLER: Since then, I've probably had a hundred people email me wanting a test. In many cases, they're people who had family members who had Alzheimer's disease and this is their biggest fear, and they want to know.

HAMILTON: Schindler says, today, doctors and their patients can choose from more than a dozen Alzheimer's blood tests.

SCHINDLER: The technology has really developed very quickly, and so there are now multiple tests that reflect the pathology of Alzheimer's disease, because people see the dollar signs. And it's very doable, to get into the market.

HAMILTON: The testing market is potentially huge because there are more than 6 million people in the U.S. with Alzheimer's and an even larger number at risk for the disease. But there's not much regulation of existing blood tests. None has been approved by the Food and Drug Administration. Schindler says that means labs are held to a less rigorous government standard.

SCHINDLER: You can have a test for Alzheimer's disease that's not very good, but as long as you get similar kind of results over time, you can market it.

HAMILTON: Late last year, the FDA announced a proposal to scrutinize tests like these more closely. Schindler says right now, the accuracy of some blood tests is less than 80% when applied to people who typically visit a dementia specialist.

SCHINDLER: With that type of test in this population, you would misdiagnose about 1-in-4 patients.

HAMILTON: But other blood tests perform much better, says Dr. Randall Bateman, a professor of neurology at Washington University in St. Louis.

RANDALL BATEMAN: The one we developed can be 95% accurate, so that's outstanding. And so now these blood tests are rivaling the performance of the PET scans and the spinal taps that we've traditionally used.

HAMILTON: Interest in blood tests has soared since July of 2023, when the FDA approved the first drug shown to slow down Alzheimer's. It's called lecanemab, and it's marketed under the brand name Leqembi. Bateman says a second Alzheimer's drug is likely to get FDA approval early this year.

BATEMAN: For the first time ever - right? - Alzheimer's doctors are now able to treat patients with these drugs.

HAMILTON: The drugs remove amyloid from the brain, and to prescribe them, doctors need to show that amyloid is present in a patient's brain. Bateman says that needs to happen quickly, while the patient is still in the early stages of the disease.

BATEMAN: There's a time window where there's a benefit. And if they're going to get treated in that time window, you almost have to have blood tests be part of the solution.

HAMILTON: Because most doctors aren't equipped to immediately offer a brain scan or a spinal tap. Bateman says it's likely that the current Alzheimer's drugs will prove most effective in patients who aren't yet showing any signs of memory impairment or thinking problems.

BATEMAN: And if that's the case, the challenge then will be, well, how do we know who to treat if they don't even have symptoms yet? And that's where the screening test will come in.

HAMILTON: The blood test would be used to screen for patients who are likely to develop Alzheimer's in the coming years. Bateman says, eventually, blood tests for Alzheimer's could even become a part of a routine doctor visit.

BATEMAN: You can imagine a scenario where someone who's, say, in their 50s, they go into their regular doctor's office for a checkup, blood pressure is checked, cholesterol is checked and a screening test for amyloid plaque is checked.

HAMILTON: When that day comes, Bateman says it will be critical to ensure that every blood test for Alzheimer's is accurate.

Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.

Jon Hamilton
Jon Hamilton is a correspondent for NPR's Science Desk. Currently he focuses on neuroscience and health risks.