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.
US scientists were able to use levels of a protein in the blood to help predict its build-up in the brain.
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.
But costly, time-consuming positron emission tomography (Pet) brain scans are currently the only way to test for this.
Researchers from Washington University School of Medicine, in St Louis, Missouri, writing in Neurology, measured levels of one protein, called amyloid beta, in the blood of 158 adults aged over 50 to see if this matched levels found in brain scans.
It did, but only 88% of the time – which is not accurate enough for a diagnostic test.
When the researchers combined this information with two other risk factors for the disease – an age of over 65 and people with a genetic variant called APOE4, which at least triples the risk of the disease – the accuracy of the blood test improved to 94%.
- Lifestyle changes that could lower your dementia risk
- New type of dementia identified
- The power of music: Vicky McClure's dementia choir
Senior study author Randall J Bateman, professor of neurology, said this could now help screen many more people than expensive brain scans.
"That means 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 as well as the human suffering that goes with it," he said.
Participants in trials must have early Alzheimer's brain changes – such as build-up of amyloid – but no cognitive problems as yet./BBC news