Scientists have developed a blood test which they believe could diagnose Alzheimer's disease in its early stages.
Experts made the breakthrough by studying blood samples from people with the disease compared with healthy subjects.
They were looking for the production of a brain hormone called dehydroepiandrosterone (DHEA).
Previous studies have shown a small but significant decrease in the amount of DHEA in the brains of people with Alzheimer's.
This decrease has been correlated to an increase in damage to specific proteins in the blood.
In the latest study, researchers from the Research Institute of the McGill University Health Centre (MUHC) in Canada based their blood test on DHEA.
They were able to promote the production of DHEA, using a chemical process called oxidation, in blood samples taken from patients without Alzheimer's.
But in the samples taken from Alzheimer's patients, oxidation did not result in an increase in DHEA.
"There is a clear correlation between the lack of ability to produce DHEA through oxidation in the blood and the degree of cognitive impairment found in Alzheimer's disease," said senior author Dr Vassilios Papadopoulos, who is director of the institute.
"We demonstrated we could accurately and repetitively detect Alzheimer's disease with small samples of blood.
"This test also allowed for differential diagnosis of early stages of Alzheimer's disease, suggesting this can be used as a test to diagnose the disease in its infancy."
Dr Papadopoulos said that, until now, there had been no way of definitively diagnosing Alzheimer's disease besides post-mortem analysis of brain tissue.
"Our clinical study shows that a non-invasive blood test, based on a biochemical process, may be successfully used to diagnose Alzheimer's at an early stage and differentiate it from other types of dementia."
He said a test was needed that would work alongside other ways of checking for Alzheimer's, such as family history of the disease, mental assessments and the physical examinations.
"An accurate, easy and specific non-invasive biochemical test that correlates with clinical findings is vital," he said.
"We believe our results demonstrate that the DHEA-oxidation blood test can be used to diagnose Alzheimer's at a very early stage and monitor the effect of therapies and the evolution of the disease."
The research was published in the Journal of Alzheimer's Disease.
Alzheimer's disease is the most common cause of dementia, affecting around 465,000 people in the UK.
Professor Clive Ballard, director of research at the Alzheimer's Society, said: "Research about the blood test for DHEA is in the very early stages and much more research is now needed to confirm these findings and to better understand whether the DHEA blood test could provide useful information about the diagnosis or progression of Alzheimer's disease.
"Combining blood tests with other promising techniques including brain scans and spinal fluid tests is likely to provide the most effective results.
"Dementia is the biggest health and social care challenge of our generation. Only with further investment in research can dementia be defeated."