A newly developed blood test can identify those at risk of Alzheimer's disease up to six years before symptoms would become apparent, researchers say.
The test looks at levels of key proteins
The test identifies changes in a handful of proteins that cells use to convey messages to one another.
The US researchers found it could indicate who had Alzheimer's, as well as who was likely to develop the condition, with 90% accuracy.
The work, led by Stanford University, features in Nature Medicine.
One of the most distressing aspects of Alzheimer's disease is the difficulty in determining whether mild memory problems are the beginning of an inevitable mental decline.
Currently, Alzheimer's is effectively diagnosed by ruling out other causes of mental decline.
Even then it can only be categorically confirmed by carrying out a post-mortem.
The latest study pinpointed a connection between changes in the brain accompanying Alzheimer's, and changes in the way cells communicate with each other.
Lead researcher Dr Tony Wyss-Coray said: "Just as a psychiatrist can conclude a lot of things by listening to the words of a patient, so by 'listening' to different proteins we are measuring whether something is going wrong in the cells."
The researchers measured levels of 120 proteins used by cells to communicate to see if any could give a clue about Alzheimer's.
Samples from five people with Alzheimer's were compared with samples from five people clear of the disease.
Levels of a number of proteins were strikingly different between the two groups.
Next, the researchers further refined their search by examining blood samples from 129 people with symptoms ranging from mild mental impairment to severe Alzheimer's.
They found that Alzheimer's was associated with specific levels of 18 key proteins.
They used this tell-tale pattern to assess a further 92 patients who had already been clinically assessed for Alzheimer's - and the diagnoses matched in nine out of 10 cases.
The test produced a similar level of success when it was used to analyse blood samples which had been taken two to six years earlier from patients, who were then followed up to find out whether their mild mental decline had progressed to something more severe.
Blood cell production
The 18 proteins that indicate Alzheimer's are also involved in the production of new blood cells, immune processes and the death of cells at the end of their natural life cycle.
Dr Wyss-Coray said: "Our hypothesis is that there is something wrong with the production of certain blood cells, which may be needed to clear that stuff that accumulates in the brain in Alzheimer's disease."
The Stanford team stressed more work was needed to confirm their results, but Satoris, a company for which Dr Wyss-Coray works as a consultant, plans to develop a prototype test for use in research labs.
Rebecca Wood, of the Alzheimer's Research Trust, said: "This is exciting research and the results and accuracy levels are very promising.
"However, further research is needed with a larger sample group to confirm these findings and determine if a simple, accurate blood test for Alzheimer's can become a reality."
Dr Susanne Sorensen, of the Alzheimer's Society, said a blood test would be a huge breakthrough.
She said: "Early diagnosis is essential if we are ever to develop treatments that can change the course or halt the progression of dementia rather than just treat the symptoms."