A blood test could identify the first signs of Alzheimer's disease and dementia 10 years before symptoms develop, scientists believe.
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Levels of a protein present in the blood are significantly lower in people who go on to develop mild cognitive impairment, a disorder that often leads to dementia, they discovered.
Researchers took samples from 100 sets of twins then followed their progress over 10 years.
As levels of the protein declined, so did their mental abilities.
Although there is no medication to treat or prevent dementia, researchers are hopeful that one day there will be, and an early test could allow it to halt the condition in its tracks. Current evidence also suggests that exercise, eating healthily and stimulating the brain can help prevent dementia's onset.
"What we do know is that prevention of the disease is likely to be more effective than trying to reverse it," says Dr Steven Kiddle, of the Medical Research Council's Social, Genetic and Developmental Psychiatry Centre at King's College London, who was the lead author.
"The next step will be to replicate our finding in an independent study, and to confirm whether or not it is specific for Alzheimer's disease, as this could lead to the development of a reliable blood test which would help clinicians identify suitable people for prevention trials," Dr Kiddle said.
The study, the largest of its kind to date, measured more than 1000 proteins in the blood to find which were most effective in predicting the onset of mental difficulties. A protein called MAPKAPK5 was lower in those whose cognitive ability declined over the period.
Dr Claire Steves, also of King's College London and a co-author of the study, said: "We're very optimistic that our research has the potential to benefit the lives of those who don't currently have symptoms of Alzheimer's, but are at risk of developing the disease."
The study was welcomed by charities.
Dr Eric Karran, director of research at Alzheimer's Research UK, said: "Accurate and early diagnosis will be essential for the development of new treatments.
"A diagnostic marker in blood could provide a valuable non-invasive and cost-effective addition to the current tests available."
The study was published in the journal Translational Psychiatry.
The London Telegraph