The blood test looks for levels of p-tau 217, a protein that acts as a signal that there are high levels of amyloid and tau building up in the brain, key markers of Alzheimer's disease. If the level of p-tau 217 is above a certain level, there is a "high probability" that the patient is suffering from the disease. If it is below a certain level, there is a low probability.
Early analysis shows that 80.6 per cent of patients with mild cognitive impairment who took the test fell into the high or low probability categories, giving doctors a tool to assess whether Alzheimer's was behind their cognitive problems.
In only 19.4 per cent of cases were the results at an "intermediate" level and inconclusive, researchers at University College London said.
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The test would be used not as a screening tool for healthy people but rather for patients "with progressive memory and thinking problems, who have been evaluated by an experienced clinician who suspects that Alzheimer's disease is a possible cause".
Professor Jonathan Schott and Dr Ashvini Keshavan of UCL are leading the study, which aims to recruit 1,100 participants who fit this bill. This number will meet guidelines for National Institute for Health and Care Excellence (Nice) to potentially approve the test for NHS use after the trials are completed in a few years.
Schott said he hoped that the test, which is already in use in his specialised clinic, would "take us a step forward in revolutionising the way we diagnose dementia".
He added: "After decades of research, we now have a blood test for Alzheimer's disease that is backed by strong scientific evidence and provides comparable information to other gold-standard diagnostic tests such as PET scans and lumbar punctures yet is far more accessible, and cheaper.
"Currently, only about 2 per cent of people diagnosed with Alzheimer's have access to one of these gold-standard diagnostic tests."
Professor Fiona Carragher, chief policy and research officer at the Alzheimer's Society, said: "Too often, dementia is diagnosed late, limiting access to support, treatment and opportunities to plan ahead."
Schott said that the machine used to do the blood tests could detect a single grain of salt in an Olympic-sized swimming pool. He said that clinicians who diagnosed patients with Alzheimer's disease were correct only about 70 per cent of the time. "What we're trying to do [with this blood test] is to increase that 70 to 90 to 95 per cent plus," he said. "That's how the trial has been designed, to show that we can make those sorts of differences."