A simple blood test that can predict the onset of Alzheimer's disease before recognisable symptoms appear could be available in two years, say scientists.
The test, likely to cost STG100-STG300 ($A180-$A550), can show with almost 90 per cent accuracy which individuals suffering from mild memory loss are going to develop Alzheimer's within a year.
It is expected to transform the search for treatments for the devastating brain illness that affects about 600,000 people a year in the UK.
To date, trials of drugs to halt or reverse Alzheimer's have all ended in failure. Therapies exist that can reduce its symptoms, but they only work for a short period of time and are not very effective.
Scientists believe a major reason for the lack of progress is that trial patients are being recruited too late, when their disease is already far advanced.
The new blood test, based on 10 "biomarker" proteins, will make it possible to test new treatments at an early stage of Alzheimer's progression.
It could also help families plan ahead and adjust to the likely prospect of one of their members being stricken by the disease.
One of the test's inventors Professor Simon Lovestone, from King's College London, said: "People come to me at the clinic because they want to know what's happening to them, and I currently can't tell them. I tell them you've got symptoms of memory loss, come back in a year's time.
"That's grim. It's horrible. You can only imagine what it's like for the patient."
Writing in the journal Alzheimer's & Dementia, the researchers describe how they investigated 26 proteins previously associated with Alzheimer's disease.
The scientists analysed blood samples from 476 confirmed Alzheimer's patients, 220 individuals with "mild cognitive impairment" (MCI) who experienced occasional memory loss, and 450 healthy elderly people.
In the vast majority of cases, memory lapses do not lead to Alzheimer's. But the researchers identified 10 blood proteins that appeared in 87 per cent of MCI patients diagnosed with the disease within a year.
Prof Lovestone said: "Alzheimer's begins to affect the brain many years before patients are diagnosed with the disease. Many of our drug trials fail because by the time patients are given the drugs, the brain has already been too severely affected.
"A simple blood test could help us identify patients at a much earlier stage to take part in new trials and hopefully develop treatments which could prevent the progression of the disease.
"The next step will be to validate our findings in further sample sets, to see if we can improve accuracy and reduce the risk of misdiagnosis, and to develop a reliable test suitable to be used by doctors."
Working with the leading UK biotech company Proteome Sciences, the scientists hope to get the test on the market in two to five years.
In practice the blood test is not expected to be used on its own, but as a first step in a diagnostic process that could also involve brain scans and taking samples of spinal fluid.
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