Blood test predicts dementia 25 years before symptoms begin

A blood test could predict dementia risk in women up to 25 years before symptoms appear, new research suggests.

Researchers from the University of California San Diego reported the findings on 10 March 2026, showing that higher levels of phosphorylated tau 217, or p-tau217, were strongly associated with future mild cognitive impairment, meaning early memory or thinking problems, and dementia.

The findings are based on data from 2,766 participants in the Women’s Health Initiative Memory Study, a large national study in the US.

The women were aged 65 to 79, cognitively healthy when they enrolled in the late 1990s and followed for up to 25 years.

Blood samples collected at baseline, meaning at the start of the study, were analysed years later to measure p-tau217, a form of tau protein linked to the brain changes seen in Alzheimer’s disease.

Over the follow-up period, women with higher levels of p-tau217 in their blood at the outset were much more likely to develop dementia later in life.

As levels of the biomarker increased, so did the risk, with women who had the highest levels facing the greatest likelihood of developing dementia over the long term.

Aladdin Shadyab, first author and associate professor of public health and medicine at UC San Diego, said: “Our study suggests we may be able to identify women at elevated risk for dementia decades before symptoms emerge.

“That kind of long lead time opens the door to earlier prevention strategies and more targeted monitoring, rather than waiting until memory problems are already affecting daily life.”

However, the researchers found the risk of cognitive impairment or dementia linked to higher p-tau217 levels was not the same for everyone.

Higher levels were more strongly associated with poorer cognitive outcomes in women over 70 than in those younger than 70 at the start of the study, and in those carrying the APOE ε4 gene, a genetic risk factor for Alzheimer’s disease.

The study also found that p-tau217 was more predictive of dementia among women who had been randomised to oestrogen plus progestin hormone therapy than among those given a placebo.

The strength of the association also differed between white and Black women, though combining p-tau217 with age improved dementia prediction similarly in both groups.

Linda McEvoy, senior author, senior investigator at Kaiser Permanente Washington Health Research Institute and professor emeritus at the Herbert Wertheim School of Public Health, said: “Blood-based biomarkers like p-tau217 are especially promising because they are far less invasive and potentially more accessible than brain imaging or spinal fluid tests.

“This is important for accelerating research into the factors that affect risk of dementia and for evaluating strategies that may reduce risk.”

Blood-based biomarkers are not currently recommended for clinical use in people without symptoms of cognitive impairment.

The authors noted that additional studies are needed to determine how p-tau217 testing might be used in routine clinical care and whether early identification can meaningfully change outcomes.

Future research will also explore how factors such as hormone therapy, genetics and age-related health conditions interact with plasma p-tau217 over the course of someone’s life to affect dementia risk.

Shadyab added: “Ultimately, the goal is not just prediction, but using that knowledge to delay or prevent dementia altogether.”