Researchers have been able to detect “markers” in the blood that identify the disease three to five years before any memory loss occurs.
The breakthrough means that treatment for the disease could be started before irreversible brain damage has been caused – dramatically slowing down progression.
The same technology could also make it easier to spot other disease such as Parkinson’s and hard-to-detect cancers, said the researchers.
Around 750,000 people in the UK suffer from dementia, more than half of whom have Alzheimer’s.
At present the only way for sure to know if someone has suffered from the condition is to carry out a brain examination after their death.
Scientists have long been trying to discover ways of detect antibodies, chemicals and other substances in the blood that identify the body is fighting the condition.
But this has proved elusive until now.
In the new study, scientists at the Scripps Research Institute in Florida, have come up with a new approach by manufacturing thousands of molecules in different shapes – known as peptoids -designed to react with antibodies.
This suggests these antibodies are unique to the condition.
Further tests on six patients and laboratory mice showed that it was 93 per cent accurate and could eventually be used to detect the condition up to five years in advance.
Professor Thomas Kodadek, who led the research, said tests on thousands more patients was needed to test the accuracy of the test.
“If this works in Alzheimer’s disease, it suggests it is a pretty general platform that may work for a lot of different diseases,” he said.
Experts described the research as exciting but cautioned that it is still in the early stages.
Dr Susanne Sorensen, head of research at the Alzheimer’s Society, said: “This is a refreshing new approach to diagnosing Alzheimer’s disease which holds promise.
“Early diagnosis is very important and a simple non-invasive blood test for Alzheimer’s disease would be invaluable.
“However, this research is in the early stages and more investigation is needed to find out it if it can be developed in to a reliable and practical test for the future.
“As a million people develop dementia in the next 10 years we urgently need more funding to find reliable tests for the diseases that cause dementia.”
Dr Bryce Vissel, head of research into neurodegenerative diseases at the Garvan Institute in Australia, said: “This exciting study has significant implications for advancing understanding and treatments of diseases like multiple sclerosis and Alzheimer’s disease.
“This is a step towards being able to develop better treatments because we may now be able to identify people early in the disease and test drugs on them before the disease is too advanced.
“Perhaps even more importantly, the study offers a new approach to identify disease markers in the blood which could have significant implications for accurately diagnosing a range of diseases.”
The research was published in the journal Cell.