Scientists have developed a blood test that could predict a woman's risk of dementia up to 25 years before symptoms appear.
The researchers say the discovery could transform how the condition is detected and treated.
Their study found that higher levels of a protein linked to the brain changes seen in Alzheimer's disease were strongly associated with future cognitive decline in otherwise healthy women.
The discovery could help doctors identify women at risk decades before symptoms develop, opening the door to earlier monitoring and prevention strategies.
Professor Aladdin Shadyab, study lead author from the University of California San Diego, said: '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.'
Such an approach would mark a major shift from current diagnostic methods, which usually rely on noticeable signs of memory loss and decline once the disease has already begun to take hold.
The study, published in the journal JAMA Network Open, followed 2,766 women aged between 65 and 79 taking part in the Women's Health Initiative Memory Study.
At the start of the research all participants were cognitively healthy.
Blood samples collected at the beginning of the study were later analysed for a protein called p-tau217.
This protein is linked to the sticky clumps and twisted fibres that build up in the brain in Alzheimer's disease. These deposits damage brain cells and disrupt the signals they use to communicate.
Alzheimer's is the most common form of dementia and is thought to account for around 60 to 70 per cent of cases.
Over the 25-year follow-up, researchers tracked which participants developed mild cognitive impairment – an early stage of memory and thinking problems – as well as dementia.
Women with higher levels of the protein in their blood at the start of the study were around three times more likely to develop dementia later in life. However, the findings also showed that risk varied between groups.
Women aged 70 or older at the start of the study who had high levels of p-tau217 were more likely to show stronger signs of cognitive decline than younger participants.
Higher levels of the protein were also more strongly linked to poor cognitive outcomes in women carrying the APOE-E4 gene – a genetic variant known to increase the risk of Alzheimer's disease.
Women on hormone replacement therapy (HRT) for menopause symptoms who also had high levels of p-tau217 were more likely to develop dementia.
Menopause has previously been linked to reductions in brain grey matter – the tissue that contains most of the brain's nerve cells and plays a key role in memory, emotions and movement – which may partly explain why women are more likely than men to develop dementia.
Higher levels of p-tau217 predicted dementia in both white and black women, but were only linked to early memory problems in white participants, suggesting different factors may drive early decline in black women.
Professor Shadyab added: 'Ultimately, the goal is not just prediction, but using that knowledge to delay or prevent dementia altogether.'
Experts not involved in the study have welcomed the findings while cautioning that more research is needed.
Professor Tara Spires-Jones, from the University of Edinburgh, said: 'This is a well conducted observational study showing that race, older age and use of hormone replacement therapy can all affect how effective a blood test is at predicting cognitive decline and dementia in women.
'Understanding how age, race and HRT might influence the interpretation of this type of blood test is important for future clinical trials.'
However, she added that the research cannot explain why HRT influences dementia risk or the effectiveness of the test.
Neurologists from the University of Oxford also described the findings as 'impressive', but stressed that not everyone with high levels of the protein will develop dementia – meaning a positive test does not guarantee the disease.
Michelle Dyson, Chief Executive Officer at Alzheimer's Society, 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.'
