The test could fill a major gap in strategies to combat brain degeneration which is thought to show symptoms only at a stage when it too late to treat effectively.
"These findings are potentially very exciting," said Simon Lovestone, a neuroscientist at University of Oxford.
"We desperately need biomarkers which would allow patients to be identified - and recruited into trials - before their symptoms begin," Lovestone added.
Neurologist Howard Federoff of Georgetown University Medical Center in Washington, DC, and his team tested participants' cognitive and memory skills and took blood samples from them - around once a year for five years.
The test was identified in a preliminary study involving 525 people aged over 70.
The work identified a set of 10 lipid metabolites in blood plasma that distinguished with 90 percent accuracy between people who would remain cognitively healthy from those who would go on to show signs of cognitive impairment.
"We do not really know the source of the 10 molecules though we know they are generally present in cell membranes," Federoff noted.
He emphasised that his results would have to be validated in independent labs and in much larger studies.
The study was published in the journal Nature Medicine.
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