Researchers say they can accurately identify people on track to develop Alzheimer's disease before symptoms appear, which could help the progress of drug trials.
UK experts said the results were promising - and a step towards a reliable blood test for Alzheimer's to speed up dementia research.
But larger studies were needed first.
Alzheimer's is the most common cause of dementia, affecting more than 520,000 people in the UK, mostly over-65s, and millions around the world.
There are currently no treatments to halt the disease.
Up to 20 years before people develop memory loss and confusion, which is characteristic of Alzheimer's, damaging clumps of protein start to build up in their brains.
Alzheimer’s blood test could predict onset up to 20 years in advance
US scientists say their blood test can be 94% effective in spotting those at risk of the illness
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