A new test can predict the risk of dementia in women up to 25 years before symptoms appear, researchers have found. Scientists believe the discovery is one of a number of new ways of spotting the cruel condition years ahead of other signs beginning to show.

The team found a blood test could identify whether someone would be likely to go on to have cognitive problems and dementia. They found higher levels of a protein called p-tau217 – which is linked to the brain changes seen in Alzheimer’s disease – were an accurate predictor of future risk.

British researchers have welcomed the research and said blood tests to pick up Alzheimer’s disease early were developing at pace. In the new study, a team from the University of California San Diego examined data for 2,766 women in the Women’s Health Initiative Memory Study, a large US study that enrolled women aged 65 to 79 in the late 1990s and followed them for up to 25 years.

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All the women had no signs of cognitive impairment at the start of the study. Blood samples collected at the start were analysed, and again years later to measure p-tau217.

Dr Aladdin Shadyab, first author on the study, 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.

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

During the 25 years of follow-up, researchers noted the women who developed memory or thinking problems, including dementia. Those who had higher levels of p-tau217 in their blood at the start of the study were much more likely to develop dementia later in life, they found.

As levels of the biomarker increased, so did dementia risk – meaning those with the highest levels generally had the biggest risk. However, the team also found that risk was not the same for everyone, with test differences seen for different ages of women, ethnicity, hormone therapy (HRT) use and genetic factors known to affect dementia - all areas which they said needed further research.

Michelle Dyson, chief executive officer at the Alzheimer’s Society in the UK, said: “Blood tests could transform how dementia is diagnosed and research we’re funding aims to make a blood test routinely available on the NHS for symptomatic Alzheimer’s disease within the next few years. This study suggests that there may be a correlation for women between having a higher level of the p-tau127 biomarker earlier in life with an increased risk of developing dementia 25 years later.

“These findings are promising, but of course more research is needed to understand whether early identification of biomarkers can affect whether people will go on to develop dementia later on. Dementia research is making huge progress and bringing great hope.

“Alzheimer’s Society will continue to invest in pioneering research to ensure people get the diagnosis, treatment and support they deserve.”

Dr Sheona Scales, director of research at Alzheimer’s Research UK, said, “Women are twice as likely to be affected by dementia, but we do not understand why there is this difference. This study adds to growing research exploring whether blood‑based biomarkers could identify who may be at higher risk of dementia decades before symptoms develop.”

Masud Husain, professor of neurology at the University of Oxford, said: “This is an impressive study that uniquely has been able to analyse blood samples from women followed up for up to 25 years. The findings show that the level of p-tau 217 in the blood provides an index of the risk of developing dementia in the future.

“These results add further support for growing evidence that testing p-tau 217 in blood might be an important way to screen for people who are most at risk of dementia, years before a diagnosis is made.” Dr Linda McEvoy, senior author of the study, 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.”