A blood test that measures a protein called p-tau217 can flag the risk of cognitive decline up to a decade before any symptoms appear.
The finding comes from a large JAMA study led by Mass General Brigham and presented at the Alzheimer's Association International Conference in London in July 2026.
Adults in the study with very high p-tau217 levels faced a 78% risk of developing cognitive impairment within 10 years, and roughly a one-in-three chance within five years. Even moderately elevated readings carried a 45% ten-year risk.
The test measures phosphorylated tau 217, a modified form of the tau protein that accumulates in the brains of people with Alzheimer's disease.
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Blood levels appear to track the buildup of amyloid and tau, the two hallmark proteins tied to the disease.
What the study actually looked at
The team pooled data from 2,684 cognitively healthy older adults, average age 70, drawn from six research cohorts in North America, Japan, and Australia. During a median follow-up of 5.4 years, 478 participants progressed to mild cognitive impairment or dementia.
Each standard-deviation rise in p-tau217 raised the hazard of progression by 38%. The signal held up even after accounting for amyloid PET scans and the APOE4 genetic variant, meaning the blood test added information beyond established risk markers.
Other factors can influence a person's baseline reading, including age, kidney function, obesity and genetics, so the biomarker cannot fully predict individual risk on its own.
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Not a diagnosis, and not for everyone
Researchers stressed that a high p-tau217 result is a risk signal, not a diagnosis.
"Very high levels of plasma p-tau217 provide meaningful information about future risk of cognitive impairment among older adults who are initially cognitively unimpaired," said lead author Rachel Buckley, a cognitive neuroscientist at Mass General Brigham Neuroscience Institute.
She added that "these findings should not yet be interpreted as support for routine screening in asymptomatic individuals." The test is currently being used in research and to help select participants for prevention trials, not to screen the general public.
Why a positive result is complicated
Even if a healthy person tested high today, there are no approved disease-modifying treatments for the pre-symptomatic stage of Alzheimer's.
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"We do not yet have disease-modifying treatments for people who find out they are at high risk," said senior author Reisa Sperling, a neurologist at Mass General Brigham.
For now, doctors point people at higher risk toward the same brain-health habits already tied to lower dementia rates. That means regular exercise, a healthy diet, adequate sleep and cognitive engagement.
Sperling's longer-term hope is that a p-tau217 blood check one day becomes as routine as a cholesterol test. Anyone worried about their memory, or with a strong family history of Alzheimer's, should talk to their doctor about whether cognitive testing makes sense.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.