Until recently, confirming that someone’s memory problems were caused by Alzheimer’s disease required either a PET brain scan, costing thousands of dollars, or a spinal tap. Most people were diagnosed on symptoms alone, and a significant share were misdiagnosed. A new generation of blood tests is changing that. Several are now cleared by the FDA. Here is what they do — and what they don’t.

What the tests measure

Alzheimer’s is defined by two abnormal proteins in the brain: amyloid, which forms plaques, and tau, which forms tangles inside neurons. These changes begin years before symptoms. Tiny traces leak into the blood. The most useful marker is p-tau217, a form of tau that rises closely in step with amyloid plaques in the brain. Some tests measure p-tau217 alone; others combine it with the ratio of two forms of amyloid (Aβ42/40) or use the related marker p-tau181. Results are reported as negative, positive or intermediate, or as a probability.

The FDA-cleared tests

  • Lumipulse G pTau217/β-amyloid 1-42 plasma ratio (Fujirebio) was the first, cleared in May 2025 for adults 55 and older with symptoms, in specialty settings.
  • Elecsys pTau181 (Roche) was cleared in late 2025 as the first blood test for primary care, specifically to rule out Alzheimer’s-related amyloid.
  • PrecivityAD2 (C2N Diagnostics) was cleared in August 2026 for adults with symptoms from age 40 (see our coverage).
  • Elecsys pTau217 (Roche), announced in August 2026, can be used both to rule in and rule out amyloid in primary and specialty care, using the same cut-offs in both.

Rule-out versus rule-in

This distinction matters. A rule-out test is designed so that a negative result reliably means amyloid plaques are unlikely, which tells a doctor to look for other causes of memory problems, such as depression, thyroid disease, vitamin deficiency, medication side effects or sleep apnea, and spares the patient further Alzheimer’s testing. A rule-in result suggests amyloid is likely present. Several tests also have an intermediate zone, where the result is uncertain and confirmation with a PET scan or spinal fluid test may be needed.

How accurate are they?

The best p-tau217 tests agree with PET scans or spinal fluid tests roughly 90% of the time in studies of people with symptoms, making them about as accurate as a spinal tap. Accuracy depends on the setting, though. In a memory clinic, where many patients have Alzheimer’s, a positive result is very likely to be correct; in primary care, where fewer do, false positives are more common. Kidney disease can raise some blood markers, and results in groups underrepresented in the validation studies are still being studied. That is why the FDA labels state that these are not stand-alone tests: results must be interpreted alongside a clinical assessment.

Who should get one

The cleared tests are for people who already have memory or thinking problems, generally aged 55 and over (40 for PrecivityAD2), being evaluated by a clinician. They are not approved to screen healthy people. Many older adults have amyloid in the brain and never develop dementia, so a positive result in someone without symptoms is hard to interpret and could cause needless anxiety. Tests sold directly to consumers without a clinician’s involvement should be approached with caution.

Why they matter now

The anti-amyloid drugs Leqembi and Kisunla can modestly slow early Alzheimer’s, but only in people with confirmed amyloid (see our comparison of the two). Blood tests make it far quicker and cheaper to find out who might be eligible, and to avoid unnecessary scans for those who are not. A clear diagnosis also helps people plan, join clinical trials and access support, even when they are not candidates for drug treatment.

What they cost and who pays

Costs vary widely and coverage is evolving. Self-pay prices quoted by labs and consumer guides range from a few hundred dollars to more than $1,000 per test, compared with several thousand for an amyloid PET scan. Medicare does not cover these tests as routine screening; coverage when a doctor orders one to investigate symptoms depends on the specific test and circumstances, and claims online about blanket coverage are not consistent. Legislation to create a Medicare coverage pathway for Alzheimer’s blood tests has been proposed in Congress but not enacted at the time of writing. Before the blood draw, ask the clinic and your insurer what will be covered and what the self-pay price is.

What a result does and doesn’t mean

  • A negative result makes Alzheimer’s unlikely as the cause of current symptoms; other causes should be explored.
  • A positive result means Alzheimer’s brain changes are likely, but does not by itself predict how quickly symptoms will progress, and should be confirmed in context.
  • An intermediate result usually means further testing.

Questions to ask your doctor

  • Which test would you use, and is it FDA-cleared for my age and situation?
  • Is it a rule-out test, a rule-in test, or both?
  • What would we do differently depending on the result?
  • Will my insurance cover it, and what is the cost if not?

Blood tests are a major step toward faster, cheaper and more accurate diagnosis. They are a tool for evaluating symptoms, not a crystal ball for healthy people. This explainer is general information, not medical advice.