By Marc Rothman, MD — Geriatrician and Founder/CEO of Lizzy Care
New blood tests can now help diagnose Alzheimer’s disease. Here’s what they can and can’t tell you.
A question I hear more and more from families these days: “Is there a blood test for dementia?”
Usually it comes from a daughter or son who has noticed something is off. Mom is repeating herself, or Dad is making errors when paying the bills. They have read a headline and want to know if a simple blood draw can tell them what’s going on.
The short answer is “yes and no”. There are blood tests that can help make a diagnosis of Alzheimer’s disease – one type of dementia – in the context of their medical history and other clinical findings. But there is no blood test for “dementia,” which has many causes. And no test replaces a careful evaluation by a clinician who knows what to look for. Still, we’re closer than ever to confirmatory testing for Alzheimer’s, and that’s real progress.
The two tests that matter right now
Researchers are studying dozens of possible blood markers, but most are still exploratory. Two are ready for the clinic, and both look for the hallmarks of Alzheimer’s in the brain: amyloid plaques and tau tangles.
The amyloid ratio test. Amyloid is the protein that clumps into plaques in the brain. When plaques build up, less of one form of amyloid (called amyloid-beta 42) reaches the bloodstream compared with another form (amyloid-beta 40). A lower ratio of the two points toward Alzheimer’s changes. This was one of the first blood markers to reach the clinic. On its own, though, the difference between a normal and an abnormal result is small, so it’s most useful when paired with a p-tau measurement.
The p-tau tests. Tau is the protein that forms tangles inside nerve cells. Certain forms of it, especially one called p-tau217, rise in the blood as Alzheimer’s develops. Of all the blood markers studied so far, p-tau217 tracks most closely with what we see on brain scans.
Some tests measure p-tau alone. Others combine it with amyloid, as a ratio. Until recently, all of them were mainly research tools, used to choose the right volunteers for drug trials. That has changed quickly. Since May 2025, the FDA has cleared four blood tests to help evaluate adults who already have memory symptoms. The most recent, cleared in August 2026, can be used in both primary care and specialty clinics.
How good are they?
In the right person with the right history and clinical picture, these tests are very good. In the study that supported the first FDA clearance of p-tau217, about 92% of people with a positive result had amyloid plaques confirmed by a PET scan or spinal fluid test. About 97% of people with a negative result did not.
Unfortunately, some results land in a gray zone, neither clearly positive nor clearly negative. In that study, it was fewer than 1 in 5. When that happens, physicians may need a PET scan or spinal fluid test to settle the question.
In 2025, the Alzheimer’s Association issued its first set of guidelines for these lab tests. The Association recommended that every test should be at least 90% sensitive to be used to rule Alzheimer’s out, and at least 90% sensitive and 90% specific to stand in for a PET scan or spinal tap. Not every test on the market meets that bar today, which is why many of us physicians don’t use them all in practice.
How I use them in practice
In my own work, I think of these tests as a way to add confidence, never as the whole answer. A diagnosis always starts with the patient’s history: what the family has noticed, how it has changed over time, and how daily life is going. Then comes a careful exam, cognitive testing, a review of medications, and lab work to rule out reversible causes of cognitive changes like thyroid problems or vitamin B12 deficiency.
A blood test fits into that picture. A negative result makes Alzheimer’s unlikely and points us toward other causes, such as vascular or Lewy body dementia, depression, sleep problems, or side effects from medication. A positive result, in someone whose symptoms fit, adds real confidence that Alzheimer’s is the correct diagnosis, which helps families focus less on diagnosis and more importantly on what can be done to stay engaged, possible therapies to reduce disease progression, and making key decisions about safety, support, and quality of life.
Clearing up the myths
Myth: “Everyone over 65 should get an Alzheimer’s blood test.” Not today. The tests are cleared for people who already have symptoms. Many older adults carry amyloid in the brain and never develop memory problems. Testing healthy people can produce worrying results that we don’t yet know what to do with.
Myth: “A positive result means Alzheimer’s explains everything.” It tells us Alzheimer’s changes are present. It doesn’t tell us they’re the only cause of symptoms. Many people have a combination of brain-related issues, including strokes, depression, or other metabolic problems.
Myth: “The number is the diagnosis.” Other health conditions can affect results. Chronic kidney disease, for example, can raise p-tau levels. A result always has to be read alongside the history and exam.
Why an official diagnosis matters
Dementia is still widely underdiagnosed. Reaching a definitive diagnosis helps families because it clears the path to resources that have been shown to help people stay vibrant and active in their normal lives.
Access to GUIDE. Medicare’s GUIDE program (Guiding an Improved Dementia Experience) gives families a dedicated care navigator, 24/7 access to help, a personalized care plan, and, for many, up to about $2,500 a year in respite support. To enroll, a person must have a dementia diagnosis and be in traditional Medicare. At Lizzy Care, we often meet people who clearly have cognitive impairment but don’t have a diagnosis, so they can’t access the benefits that Medicare created just 2 years ago. So we help them confirm diagnoses whenever we can.
Access to newer medications. The anti-amyloid drugs lecanemab (Leqembi) and donanemab (Kisunla) are approved for people in the early stages of Alzheimer’s. Before anyone starts, we must confirm amyloid is present. Blood tests are becoming part of that workup, though many clinicians still confirm with a PET scan or spinal fluid test.
Time to plan. Just as important, a diagnosis gives families time: to talk about wishes while the person can still take part, to sort out legal and financial matters, and to build a care team before a crisis hits.
Are there downsides to testing?
Once symptoms have surfaced, I believe the benefits of testing usually outweigh the risks. Still, families should go in with open eyes.
The road ahead
Ten years ago, confirming Alzheimer’s during life meant a PET scan few people could get or a spinal tap few people wanted. Today, a blood draw can bring us most of the way there. As the tests improve, I expect earlier and more accurate diagnoses to become the norm rather than the exception.
My advice for now: if memory or thinking changes have surfaced, don’t wait. Ask your doctor whether a blood test belongs in a complete evaluation. And find a trusted team to help you make sense of the results and the road ahead. There are many forks along the dementia highway, and no one should have to navigate them alone.
Reach out, and take care.
Dr. Marc Rothman has dedicated his professional career to helping older adults. As CEO of Lizzy Care, a full-service dementia care management company, Dr. Rothman draws from his years as a practicing physician and physician executive to design programs that deliver care more safely and effectively. Dr. Rothman is also the founder of Dementia Spring Foundation, a 501(c)(3) nonprofit that supports artists and innovators telling new stories of dementia, and advocates for the visual and performing arts as safe, effective treatments for reducing caregiver stress, behavioral symptoms of dementia, and improving quality of life. |
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