A test which can spot Alzheimer's years before full-blown symptoms arise has been hailed as potentially a "real weapon" against the disease.
It involves examining levels of two particular proteins in spinal fluid, that tend to be higher in people who go on to develop the brain disorder.
Alzheimer's is often preceded by a condition called mild cognitive impairment (MCI), which involves memory and thinking problems.
About 15pc of people with MCI go on to be diagnosed with clinical Alzheimer's every year.
However, some regain their full brainpower, but doctors do not know who will go down which route or why.
Now scientists have found that by looking at concentrations of two proteins in spinal fluid together, they are able to tell in four out of five cases who will go on to develop Alzheimer's and who will not.
Researchers at the Technical University Munich analysed a number of spinal fluid proteins in 58 people with MCI, 21 of whom went on to be diagnosed with Alzheimer's over three years.
They found that two proteins - one called tau and another called soluable amyloid precursor protein beta (sAPP?) - were particular powerful predictors when combined with a person's age.
Tau, which causes cell death when it builds up in the brain, is already well established as a biomarker. The other, sAPP?, is a new finding.
In a paper published last night (TUES) in the journal Neurology, the authors wrote that the new protein was of great value because levels became raised very early during disease development.
The Alzheimer's Research Trust, a charity, commented that spinal fluid protein tests "might be real weapons" in tackling the disease.
Rebecca Wood, its chief executive, added: "This small study provides a potential new lead to follow up."
Prof Simon Lovestone, of the Institute of Psychiatry at King's College London, said: "If this finding is replicated in larger studies it would be a significant step forward."
Adding the new protein made predictions more accurate, which was "really important," said Prof Lovestone, one of Britain's leading experts on Alzheimer's biomarkers.
He said that perfecting a predictive test for Alzheimer's was essential to develop drugs to combat the disease.
The was because changes began in the brain up to a decade before clinical symptoms emerged, he explained.
Secondly, if people knew they were very likely to get Alzheimer's they could plan for the future.
"You might go ahead with that world cruise and think about what care you want, if you know," he said.
But he cautioned that all spinal fluid tests had an Achilles' heel - they required an "uncomfortable" lumbar puncture to obtain the sample.
"That's particularly problematic if you want to give the test lots of times, for example at three or six month intervals."
Prof Lovestone is currently working on a blood test to detect the early signs of Alzheimer's, a disease which currently affects more than 800,000 people in Britain.