A simple blood test could be developed to predict Alzheimer's disease up to a decade before symptoms appear, scientists claim.

A breakthrough by British researchers has identified a single blood protein that acts as a warning for mild cognitive impairment - a disorder that is often the precursor to dementia.

In the largest study of its kind, the researchers monitored more than 1,100 proteins in the blood of 106 pairs of twins. Tracking them over ten years, they found that those whose thinking skills diminished the most had lower levels of an individual protein.

The research is at an early stage, but scientists hope it might be developed into a test that flags up those at risk of developing dementia. There are currently no treatments proven to prevent Alzheimer's but doctors hope that identifying those most at risk could speed the search for new drugs that could delay or even prevent the devastating brain disease.

Flagging up those at risk would give patients and their families more time to prepare, they hope.

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The protein - called MAPKAPK5 - was, on average, lower in individuals whose cognitive ability declined over a ten-year period. By studying identical twins - who share their genes - the scientists showed that the association between the protein and cognition was independent of age and genetics.

Alzheimer's patients are currently diagnosed only when they start to lose their memory - and thousands are thought to be living without a diagnosis. Brain scans have been shown to display visible signs of the disease before the onset of symptoms but they are expensive.

Study author Dr Steven Kiddle, of King's College London, said: "Although we are still searching for an effective treatment for Alzheimer's disease, what we do know is that prevention of the disease is likely to be more effective than trying to reverse it.

"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 Claire Steves, senior lecturer in twin research at King's College London, added: "We're very optimistic that our research has the potential to benefit the lives of those who don't have symptoms of Alzheimer's, but are at risk of developing the disease."

Charities welcomed the findings, which were funded by the Medical Research Council and published in the journal Translational Psychiatry.

Dr Eric Karran, of Alzheimer's Research UK, said: "Searching for reliable markers of disease in the blood is a tricky task, as protein levels can be influenced by so many factors.

"Twin studies present a unique insight into the biology of complex diseases like Alzheimer's, as they control for age and most genetic effects.

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"This study associated blood levels of MAPKAPK5 with cognitive decline over a ten-year period, but it will be necessary to investigate more about a possible mechanism linking this protein to changes in memory and thinking. Accurate and early diagnosis of Alzheimer's will be essential for the development of new treatments, allowing clinical trials to take place."

- Daily Mail