BOSTON — The secret might be hidden in our blood.

Boston researchers in a new study have found that a blood test can predict Alzheimer’s disease progression years before symptoms or brain scan changes.

The Mass General Brigham scientists discovered that a blood test of plasma phosphorylated tau 217 (pTau217) can detect the earliest signals of Alzheimer’s disease in cognitively healthy adults.

The blood test could help identify who may be at risk for cognitive decline, and help “push back the clock to enable earlier Alzheimer’s disease prediction.”

“We used to think that PET scan detection was the earliest sign of Alzheimer’s disease progression, revealing amyloid accumulation in the brain 10 to 20 years before symptoms appear,” said lead author Hyun-Sik Yang, a neurologist with Mass General Brigham Neuroscience Institute.

“But now we are seeing that pTau217 can be detected years earlier, well before clear abnormalities appear on amyloid PET scans,” added Yang, who’s also an associate member of the Broad Institute of MIT and Harvard.

Last year, the FDA cleared the first blood test for Alzheimer’s disease, paving the way for a cheaper, less invasive alternative to lumbar punctures and PET scans.

The new Mass General Brigham study adds important evidence about the predictive potential of these kinds of blood tests.

This prospective cohort study followed 317 cognitively healthy older adults from the Harvard Aging Brain Study for an average of eight years.

Participants — who ranged in age from 50 to 90 years — had blood tests for pTau217, repeated amyloid and tau PET scans, and long-term cognitive testing.

The researchers examined whether baseline and changing pTau217 levels predicted future amyloid buildup, tau accumulation (the abnormal buildup of misfolded tau proteins inside brain neurons), and cognitive decline.

Scientists found that higher levels of pTau217 predicted a faster buildup of Alzheimer’s disease pathology, even when initial brain scans appeared normal. Increases in pTau217 frequently happened before amyloid PET scans became positive — highlighting the biomarker’s ability to detect changes earlier.

Importantly, participants with low pTau217 levels at the start of the study were very unlikely to accumulate significant amyloid-beta on their PET scans over many years of follow-up.

“What stood out in our study is that even when amyloid scans appear normal in the clinic, the pTau217 biomarker can identify individuals who later become amyloid-positive,” Yang said. “It also shows that those with low pTau217 levels are likely to stay amyloid-negative for several years.”

While it’s too early to recommend pTau217 testing for older adults, Yang and his colleagues hope the study results will serve as a scalable screening tool for clinical trials targeting Alzheimer’s disease prevention, and will help identify individuals at higher risk.

Eventually, biomarker blood tests could be used for routine health maintenance, and may offer a more affordable alternative to amyloid PET scans.

“As the field is evolving quickly, we’re excited to see discoveries on the research side being rapidly translated to clinical application,” said co-senior author Jasmeer Chhatwal, a neurologist with Mass General Brigham Neuroscience Institute. “By anticipating who’s going to turn amyloid-positive in the future, we are trying to push back the clock to enable earlier Alzheimer’s disease prediction.”

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