Understanding P-tau217 and Blood-Based Alzheimer Biomarkers

Blood-based biomarkers have rapidly emerged as one of the most significant advances in Alzheimer's disease diagnostics, offering the potential to detect underlying pathology with far greater accessibility than traditional cerebrospinal fluid analysis or amyloid PET imaging. As clinicians prepare for an era of earlier diagnosis and disease-modifying treatment, understanding what these biomarkers measure and how they should be interpreted has become increasingly important.

At the 2026 Alzheimer's Association International Conference (AAIC), investigators presented data demonstrating that P-tau217 blood-based assays achieved strong rule-in performance for identifying Alzheimer's pathology in cognitively unimpaired individuals, with findings that were noninferior to amyloid PET across multiple analyses. These results further support the growing role of blood-based biomarkers as scalable diagnostic tools capable of identifying amyloid pathology before overt clinical decline.

In this episode of the NeurologyLive^® Special Report, Yogesh Shah, MD, MPH, FAAFP, medical director of the Broadlawns Memory Clinic, site principal investigator for the University of Iowa Geriatric Workforce Empowerment Program, and board member of the Iowa Chapter of the Alzheimer's Association, explains the biology behind P-tau217, reviews how the biomarker relates to amyloid and tau pathology, and discusses why blood-based testing represents a meaningful step toward more accessible and earlier Alzheimer's disease diagnosis.

Transcript edited for clarity.

Yogesh Shah, MD, MPH, FAAFP: Looking at the pathology of Alzheimer disease, it starts with a protein called amyloid, which is found outside of our brain cells. We all have a normal protein called amyloid precursor protein, or APP. It's a long protein made up of about 770 amino acids that is present in all of us.

Over time, APP is broken down into smaller fragments known as amyloid beta 42 and amyloid beta 40. Amyloid beta 42 is the sticky form. It aggregates with itself and forms amyloid plaques outside of brain cells. As those plaques accumulate, they trigger changes inside the neurons, where the normal tau protein becomes phosphorylated. The tau protein then loses its normal function, clumps together within the cell, and forms what we know as neurofibrillary tangles.

So now we have two major proteins involved in Alzheimer disease: amyloid outside the brain cells and tau inside the brain cells.

For years, we've been trying to determine the best way to measure these proteins. One approach has been lumbar puncture, where cerebrospinal fluid is collected and analyzed for amyloid beta 42, amyloid beta 40, and tau. While it's an excellent test, it requires a procedure, scheduling, and carries potential complications.

The other established approach is amyloid PET imaging, where a radioactive tracer binds to amyloid plaques in the brain, allowing us to visualize amyloid burden. Those scans quantify amyloid using a measurement called centiloids.

More recently, over the past several years, we've developed blood-based biomarkers. Initially, there was the amyloid beta 42 to 40 ratio, and now we have P-tau217, which measures phosphorylated tau at amino acid 217. This biomarker correlates very closely with both amyloid PET imaging and cerebrospinal fluid testing.

The version of P-tau217 that I use through the Mayo Clinic has approximately 92% sensitivity and 96% specificity, making it a highly accurate test with a very small chance of error. It uses a double-cutoff approach, classifying results as negative, positive, or intermediate.

I often compare it to a traffic light. Green means the test is negative and the likelihood of Alzheimer pathology is very low. Red means the result is positive, above the established cutoff, and strongly suggests amyloid pathology in the brain. Yellow represents an intermediate zone where additional testing, such as an amyloid PET scan or lumbar puncture, is needed to confirm the diagnosis.

The excitement surrounding P-tau217, and one of the reasons it's generated so much discussion here at AAIC 2026, is that we can now identify Alzheimer pathology much earlier than ever before. The hope is that, one day, we'll be able to detect these changes years before symptoms develop and intervene through strategies that reduce a person's future risk. There's already a growing body of evidence supporting Alzheimer disease risk reduction.

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