A new blood test that can identify signs of Alzheimer’s disease decades before symptoms appear is 94 percent accurate.
Researchers say that checks could be available at GP surgeries "within a few years".
At present its main benefit will be to help understand the development of the disease, for which there is as yet no cure. But it will also identify people who can take part in clinical trials of new drugs.
The test identifies the build-up of damaging clumps of protein in the brain, a hallmark of the disease, long before the characteristic memory loss and confusion of Alzheimer’s set in.
Researchers at Washington University School of Medicine in the US were able to measure levels of the protein amyloid beta in the blood and predict whether it had accumulated in the brain.
When combined with two other major risk factors - age and the gene APOE4 - people with Alzheimer’s brain changes can be identified with 94 percent accuracy.
The team said the findings, published in the journal Neurology, represent a step toward a blood test to predict which people are likely to develop Alzheimer’s.
Study senior author Professor Randall Bateman said: "With a blood test, we can more efficiently enrol participants in clinical trials, which will help us find treatments faster, and could have an enormous impact on the cost of the disease."
Dr Sara Imarisio, head of research at Alzheimer’s Research UK, said: "An accurate and reliable blood test for Alzheimer’s could revolutionise dementia research."