Proteins in blood may help detect if a person is suffering from dementia a decade before they experience troubling symptoms, scientists have said.
For the study, blood samples were taken from 20 people who later developed Alzheimer's disease, up to 10 years before they were diagnosed and then after they were diagnosed.
Blood was also taken once from 26 people with Alzheimer's disease and 16 people with frontotemporal dementia, which leads to changes in personality or behaviour, and also may affect the memory.
Samples were also taken from 46 healthy people who did not have any problems with thinking or memory skills.
American scientists found the level of protein was significantly different for the healthy controls than for those with dementia, both before and after symptoms developed.
The findings are published in Neurology, the medical journal of the American Academy of Neurology.
Gordon Wilcock, emeritus professor of clinical geratology at the University of Oxford, said: "This is important as it suggests that altered blood proteins may be a marker of early Alzheimer's disease, possibly 10 years before significant memory problems develop.
"It also gives us further insight into what might be going wrong in brain cells. However, it is too soon to consider the results a breakthrough.
"These findings need to be checked in a larger number of subjects to be certain that the changes are sufficiently sensitive and specific to be used as a clinic test, and also whether they can predict which asymptomatic people will develop dementia in due course."
Dr Eric Karran, from Alzheimer's Research UK, said: "This small study is one of an emerging research area that might hold real promise for early diagnosis of some neuro-degenerative diseases.
"It needs to be confirmed in much larger groups of patients to ensure the findings reported are robust.
"The ability to accurately identify Alzheimer's at the earliest stages would be a crucial step forward for research, as it's likely that new treatments will have the best chance of success if given early.
"If confirmed in larger studies, these findings could enable the right people to be recruited for clinical trials, but this test is not designed for use in the doctor's clinic.
"Research like this can also provide valuable clues about changes inside brain cells during Alzheimer's, helping to guide efforts to develop new and effective treatments."
More than half a million people in the UK are affected by Alzheimer's.