A simple blood test that identifies people at risk of Alzheimer's could become a reality after the discovery of new telltale biomarkers.
Scientists isolated protein fragments from blood samples that allowed them to predict brain changes associated with the disease.
If further studies confirm the test's effectiveness it could be used to spot early signs of Alzheimer's in apparently healthy individuals.
Experts agree that new treatments designed to halt or reverse the disease are only likely to work if given early enough.
The changes that lead to Alzheimer's are thought to take root up to 20 years before symptoms appear.
Alzheimer's is the most common form of dementia, accounting for around 62% of cases.
There are 850,000 people with dementia in the UK, with numbers set to rise to more than one million by 2025.
A total of 225,000 Britons will develop dementia this year, one every three minutes.
More than 40,000 people under the age of 65 suffer from dementia in the UK.
One in six people over the age of 80 have dementia.
Seventy per cent of people in care homes have dementia or severe memory problems.
Delaying dementia onset by five years in the UK would save 30,000 lives a year.
Around the world there are an estimated 46.8 million people living with dementia. The number is expected to rise to more than 115 million in 2050.
Alzheimer's is strongly linked to the build-up of clumps of toxic beta-amyloid peptide, a protein building block, in the brain.
Currently levels of the peptide can only be measured in living patients using costly brain scans or highly invasive tests of cerebrospinal fluid.
The new blood plasma biomarkers consist of several peptide fragments associated with beta-amyloid.
A Japanese team led by Dr Katsuhiko Yanagisawa, from the National Centre for Geriatrics and Gerontology in Obu, showed that relative levels of the biomarkers reflected the state of beta-amyloid deposition in the brain.
The test was assessed using samples from 373 Japanese and Australian participants including mentally normal individuals as well as people with mild cognitive impairment and Alzheimer's.
The scientists wrote in the Nature journal: "All test biomarkers showed high performance when predicting brain amyloid-beta burden.
"These results demonstrate the potential clinical utility of plasma biomarkers in predicting brain amyloid-beta burden at an individual level."
Commenting on the results, British experts said the research was promising but more work needed to be done.
Dr Carol Routledge, from the charity Alzheimer's Research UK, said: "A blood marker for Alzheimer's could prove valuable as a minimally invasive and cost-effective addition to the current diagnostic tests available for the disease.
"The ability to detect the very earliest stages of Alzheimer's would not only allow people to receive existing treatments sooner but would also enable new drugs to be trialled in the right people, at the right time."