A test assessing biochemical changes in the brain may help identify people who are at risk of developing Alzheimer's disease.
Research published in Neurology, the journal of the American Academy of Neurology, identified significantly high levels of plaques on the brains of 33 per cent of participants in their 70s and 80s who had no cognitive problems.
Such plaques are one of the major signs of Alzheimer's and while long-term study would be required to see how at risk the subjects are, the team suggested it could indicate some are in the very early stages.
Jonathan Schott of the Dementia Research Centre at University College London noted the advanced brain imaging technique used could help identify changes that take place years before symptoms actually develop.
And this may create a "window of opportunity" to prevent or delay memory loss and cognitive decline as new treatments become available, he said.
According to the Alzheimer's Society, there are 750,000 people with dementia in the UK, with the figure expected to pass one million in the next ten years.