An Alzheimer’s Blood Test Held Up Well Against the Spinal-Fluid Standard

CATEGORY: BRAIN & MENTAL HEALTH

A simple blood test correctly identified whether Alzheimer’s disease was present or absent in about 85% to 91% of people being evaluated for memory problems, in a study of 1,767 patients at clinics in Sweden, Spain and Italy.¹ In an earlier study by the same group, a similar blood test was about 90% accurate—compared with 61% for primary care doctors and 73% for dementia specialists making the call the traditional way, with an exam, memory testing and a brain scan.² Until recently, confirming Alzheimer’s required a spinal tap or a specialized brain scan that most people never receive.

The Test Measures a Tau Fragment Called p-tau217

The test measures a protein fragment in the blood called p-tau217—a form of tau, one of the two proteins that build up abnormally in the brains of people with Alzheimer’s. As tau begins tangling inside brain cells, small amounts of this fragment leak into the bloodstream, like smoke escaping under a door: it signals what is happening inside without opening the room. The new study used a fully automated, commercially available version that ordinary hospital laboratories can run.¹

Blood Results Judged Against Spinal Fluid in 1,767 Patients

Each of the 1,767 participants had come to a doctor with memory or thinking complaints; about 1,200 were seen at specialist memory clinics and 548 at ordinary primary care practices. Every participant gave a blood sample and also had a spinal-fluid test—the established laboratory standard for detecting Alzheimer’s changes—and the blood result was judged against it, using cutoff values fixed in advance. Roughly half the participants turned out to have Alzheimer’s changes.¹

Two Cutoffs Raised Accuracy to 92% to 94%

In the specialist clinics, the blood test agreed with the spinal-fluid result 89% to 91% of the time; in primary care, 85% of the time. Accuracy slipped to 83% in people aged 80 and older. Using two cutoffs instead of one—a clearly negative zone, a clearly positive zone, and a gray zone between—raised accuracy to 92% to 94%, though 12% to 17% of people landed in the gray zone and would need further testing.¹ A mass-spectrometry version of the test matched the automated one in specialist clinics but was more accurate in primary care and unaffected by age.¹ The 2024 study of 1,213 Swedish patients found a similar blood test roughly 90% accurate, well above the 61% and 73% of primary care doctors and specialists working without it.²

Thyroid Problems and Depression Can Also Cause Memory Trouble

The test was studied only in people who already had symptoms; it has not been shown useful in people without them. A positive result indicates that Alzheimer’s-type changes are likely present, not how much they explain a person’s symptoms—thyroid problems, depression and medication effects can also cause memory trouble, and some people carry Alzheimer’s changes for years without developing dementia. Roughly one result in ten was wrong in either direction, so the authors describe the test as a way to streamline diagnosis, not replace clinical judgment. U.S. regulators cleared the first blood test of this type for clinical use in 2025.

30 Studies Pooled, 82% Flagged and 86% Cleared

Independent research points the same way. A 2025 review pooling 30 studies found blood p-tau217 correctly flagged about 82% of people with Alzheimer’s-type protein changes and correctly cleared about 86% of those without, with accuracy strongest in people who already had memory complaints.³ A separate Spanish study of 493 people, using the same automated platform, reported 92% and 91% on those same two measures against spinal-fluid results.⁴

Key Takeaways

• An automated p-tau217 blood test identified Alzheimer’s changes with 85% to 91% accuracy in 1,767 people with memory complaints, checked against the spinal-fluid standard.
• In an earlier study, a similar blood test was about 90% accurate versus 61% for primary care doctors and 73% for specialists using exams, memory tests and brain scans.
• Accuracy was somewhat lower after age 80, about one in ten results was wrong, and the test was studied only in people who already had symptoms.

References

1. Palmqvist S, Warmenhoven N, Anastasi F, et al. Plasma phospho-tau217 for Alzheimer’s disease diagnosis in primary and secondary care using a fully automated platform. Nature Medicine. 2025;31(6):2036–2043. link
2. Palmqvist S, Tideman P, Mattsson-Carlgren N, et al. Blood Biomarkers to Detect Alzheimer Disease in Primary Care and Secondary Care. JAMA. 2024;332(15):1245–1257. link
3. Khalafi M, Dartora WJ, McIntire LBJ, et al. Diagnostic accuracy of phosphorylated tau217 in detecting Alzheimer’s disease pathology among cognitively impaired and unimpaired: a systematic review and meta-analysis. Alzheimer’s & Dementia. 2025;21(2):e14458. link.
4. Martínez-Dubarbie F, Guerra-Ruiz A, López-García S, et al. Diagnostic performance of plasma p-tau217 in a memory clinic cohort using the Lumipulse automated platform. Alzheimer’s Research & Therapy. 2025;17(1):68. link.