Blood Test for Alzheimer's: How Plasma Biomarkers
Quick Facts
Why Is a Blood Test for Alzheimer's a Major Breakthrough?
For decades, definitive Alzheimer's diagnosis required either an amyloid PET scan, which is expensive and not widely available, or a lumbar puncture to measure cerebrospinal fluid biomarkers. Both procedures created significant bottlenecks, especially in primary care and rural settings, leaving many patients undiagnosed or diagnosed only late in the disease course. The arrival of validated plasma biomarker assays — particularly tests measuring phosphorylated tau 217 (p-tau217) in combination with amyloid beta 42 — has changed the diagnostic landscape.
In May 2025, the U.S. Food and Drug Administration cleared the first blood-based diagnostic for Alzheimer's disease, an in vitro test measuring the plasma p-tau217 to amyloid beta 1-42 ratio. According to the FDA announcement, the assay is intended for adults aged 55 and older presenting with cognitive symptoms, helping clinicians identify amyloid pathology associated with Alzheimer's. The clearance marks a turning point because it gives community physicians a practical tool to triage patients, decide who should be referred to a specialist, and identify candidates for newly approved anti-amyloid therapies.
How Do P-Tau217 and Amyloid Biomarkers Detect Alzheimer's?
Alzheimer's disease is defined pathologically by extracellular amyloid plaques and intracellular tau tangles. Phosphorylated tau at threonine 217 (p-tau217) is a soluble form of tau that increases in plasma in close correlation with brain amyloid burden, making it a sensitive early indicator. Amyloid beta 42, by contrast, decreases in plasma as more of it becomes trapped in plaques. Combining these two markers as a ratio improves diagnostic performance compared with either alone.
Multiple peer-reviewed studies in journals such as JAMA Neurology and Nature Medicine have reported that p-tau217-based assays show strong agreement with amyloid PET imaging, often performing comparably to cerebrospinal fluid markers. Research groups including those associated with Lund University's BioFINDER program have been central to validating these markers across diverse cohorts. The clinical implication is significant: a primary care physician can now order a blood test that, when interpreted alongside a thorough cognitive assessment, provides reliable evidence of underlying Alzheimer's pathology.
What Does Earlier Diagnosis Mean for Alzheimer's Treatment?
Disease-modifying anti-amyloid antibodies — lecanemab (Leqembi) and donanemab (Kisunla) — are approved for early symptomatic Alzheimer's, including mild cognitive impairment due to Alzheimer's and mild dementia. Both require confirmation of amyloid pathology before treatment can begin. Without affordable diagnostic infrastructure, many eligible patients have been unable to start therapy in the narrow window when it offers the greatest benefit. Plasma biomarker testing helps close that gap.
According to the World Health Organization, roughly 55 million people worldwide live with dementia, with Alzheimer's accounting for the majority of cases. The Alzheimer's Association estimates approximately 7 million Americans currently have Alzheimer's, a figure projected to rise sharply with population aging. Scaling blood-based diagnostics could shorten the diagnostic odyssey, reduce health system costs, and enable population-level identification of candidates for emerging therapies and prevention trials. Researchers caution, however, that blood tests should not be used as standalone screening in asymptomatic adults; current guidance from the Alzheimer's Association supports their use in symptomatic patients evaluated by qualified clinicians.
Frequently Asked Questions
Availability is expanding rapidly following the FDA's 2025 clearance of the first plasma Alzheimer's diagnostic. Specialty memory clinics and many academic centers offer p-tau217-based testing, and primary care access is growing. Testing is intended for adults with cognitive symptoms, not as a general screening tool for healthy individuals.
No. A positive plasma biomarker test indicates evidence of Alzheimer's pathology — typically amyloid in the brain — but symptoms, severity, and progression vary widely. Results must be interpreted by a clinician alongside cognitive assessment, medical history, and sometimes imaging. Many people with biomarker changes remain stable for years.
Studies show that high-performing p-tau217 assays achieve agreement with amyloid PET in the range typical for clinical diagnostic tests. PET remains the imaging gold standard for visualizing plaques, but blood testing is far less expensive, more accessible, and avoids radiation exposure, making it suitable for first-line use in symptomatic patients.
Lecanemab and donanemab require confirmation of brain amyloid before treatment. Blood biomarker testing is increasingly accepted as part of that diagnostic workup, often used to triage which patients should proceed to confirmatory imaging or treatment evaluation, depending on local clinical protocols.
References
- U.S. Food and Drug Administration. FDA Clears First Blood Test Used in Diagnosing Alzheimer's Disease. May 2025.
- World Economic Forum. Recent breakthroughs in the fight against Alzheimer's disease. 2026.
- World Health Organization. Dementia Fact Sheet.
- Alzheimer's Association. 2025 Alzheimer's Disease Facts and Figures.
- JAMA Neurology. Studies on plasma p-tau217 for Alzheimer's diagnosis.
- Lund University BioFINDER Study. Plasma biomarker validation research.