A marker that shows up early
A blood test measuring phosphorylated tau 217, or p-tau217, can help predict which older adults with normal memory will develop cognitive impairment, according to an international study led by the Mass General Brigham Neuroscience Institute. The work combined six observational studies and clinical trials in North America, Japan and Australia, covering 2,684 people who showed no signs of impairment when they enrolled.
Participants gave blood samples at the start and were followed with annual assessments of memory and thinking. About 478 of them eventually developed cognitive impairment. Some entered the studies as early as 2004, and the most recent follow-ups took place in 2025.
Risk rises steeply with biomarker levels
Among people with very high p-tau217 levels, the estimated probability of developing cognitive impairment was 38% within five years. Over ten years the estimate rose to 78%, although the researchers cautioned that far fewer observations were available beyond the five-year mark, making the longer projection less certain.
The association held after the team accounted for amyloid-beta plaques seen on PET scans and for genetic risk such as APOE4. That matters because it suggests the blood marker is picking up something the existing tests miss, rather than repeating them.
What it could change in the clinic
A test for p-tau217 recently received federal clearance in the United States, which makes the question of how to use it practical rather than theoretical. Doctors could use results to decide who needs brain imaging, who should join prevention trials, and who should be monitored more closely.
The researchers say the findings are not a diagnosis. A high reading means higher risk, not a certainty, and a low reading does not rule out other causes of memory loss. They also note that the study population was mostly older adults who volunteered for research, which may not match the general population.
The results were presented at the Alzheimer's Association International Conference and published at the same time in JAMA. Independent researchers called the sample size and follow-up length notable, while warning that blood tests still need clear guidance on how results should be explained to patients.