A blood test measuring a specific protein may help identify healthy older adults at high risk of developing Alzheimer’s disease symptoms within the next five to ten years, according to research published Wednesday in JAMA and presented at the Alzheimer’s Association International Conference in London.
The study found that apparently healthy older adults with very high levels of a biomarker called phosphorylated tau 217, or p-tau217, had a 38 percent risk of developing cognitive impairment over five years and a 78 percent risk over ten years. Those with very low levels had correspondingly low risks of developing cognitive problems in that timeframe.
The findings come from an international research team led by experts from the Mass General Brigham Neuroscience Institute who analyzed data from 2,684 older adults across six different study cohorts. The researchers tracked participants starting as early as 2004, giving them p-tau217 blood tests at enrollment and conducting yearly cognitive checkups. Between the earliest enrollments and last year, about 478 of those participants developed cognitive impairment, which was defined as mild cognitive impairment, dementia, or other measurable cognitive decline.
The p-tau217 test measures a form of tau protein that correlates with brain amyloid plaques and provides information about tau tangles—both hallmarks of Alzheimer’s disease. Alzheimer’s is characterized by a buildup of amyloid-beta plaques and tau tangles in the brain that damage and kill nerve cells, leading to memory loss and cognitive decline. The connection between these proteins and cognitive problems has long been a focus of Alzheimer’s research, but understanding when and how they translate into symptoms has remained challenging.
One of the key puzzles in predicting Alzheimer’s involves the fact that many people develop high levels of amyloid plaques in their brains yet never go on to develop dementia. Current theory suggests that at some critical point, amyloid buildup triggers an abnormal type of tau to form tangles, which leads to symptoms. The new blood test findings offer clues about identifying that tipping point.
The results showed that higher p-tau217 levels consistently predicted cognitive decline across the six different study populations, and this relationship held even after accounting for amyloid positron emission tomography (PET) scans, which directly visualize amyloid buildup in the brain. The association was also independent of known genetic risk factors like the APOE4 gene variant.
However, the research team and independent experts emphasized important limitations. The predictions are based on selected research cohorts and represent group-level statistics rather than precise individual forecasts. Scientists stress that the test is not yet accurate enough to guide decisions for individual patients and should not be used for healthy people without symptoms. The blood test is currently used to help confirm Alzheimer’s diagnosis in people already experiencing cognitive problems, not to screen asymptomatic individuals.
Additional factors could influence predictions, experts noted. Older adults face competing health risks—they may develop vascular dementia from heart problems or stroke rather than Alzheimer’s disease, or they may die from other causes before developing cognitive impairment. Other neurodegenerative conditions or vascular issues could also lead to cognitive decline that isn’t related to Alzheimer’s pathology.
The potential clinical utility of such tests hinges on whether drugs can prevent or delay cognitive decline in people at high risk. Large clinical trials are currently testing whether certain medications can prevent or slow Alzheimer’s in its earliest stages. If successful, the blood test could help identify which healthy people would benefit most from those preventive treatments.
For now, the blood test represents a research and diagnostic tool rather than a screening test for the general public. Experts recommend waiting until preventive therapies are available before seeking out p-tau217 testing as a healthy person.
The research underscores how blood biomarkers are transforming Alzheimer’s research and clinical practice. Where traditional diagnostic approaches like spinal fluid analysis and brain imaging are invasive, expensive, or difficult to access, blood tests offer a simpler, more scalable alternative. The p-tau217 test was recently approved by the FDA for diagnostic use, though it is not yet covered by most insurance plans.
The findings add to growing evidence that blood-based biomarkers can detect Alzheimer’s-related brain changes years before symptoms appear. Earlier studies have shown that such biomarkers can identify pathological changes 15 to 20 years before clinical symptoms emerge. This extended window offers potential opportunities for early intervention, though researchers emphasize that more work is needed to validate these tests across diverse populations and to confirm that treating people in these early stages can prevent or delay disease progression.