A multi-tasking test can help avoid confusion between symptoms of depression and early Alzheimer's, research suggests.
People developing Alzheimer's suffer from mild levels of impaired reasoning and memory that are easily mistaken for signs of depression.
As a result many patients with the dementia illness are misdiagnosed and fail to receive early treatment that could help.
One way to tell the conditions apart is to ask patients to perform two mental tasks at the same time, scientists have now found.
A team led by Professor Sergio Della Sala, from the University of Edinburgh, compared the "dual-tasking" ability of 89 Alzheimer's patients, sufferers of chronic depression and healthy elderly individuals with no memory impairment.
The findings, reported in the Journal of Neurology, showed that people with Alzheimer's performed significantly worse than the other two groups. This was true even when allowances were made for individual memory differences.
Around 700,000 people in the UK have dementia, and more than half suffer from Alzheimer's.
Dr Susanne Sorensen, head of research at the Alzheimer's Society, which funded the study, said: "This is the first piece of research to compare the performance of dual tasks in Alzheimer's disease and depression and could mean that people with dementia are diagnosed earlier.
"Currently, up to two thirds of people with dementia never receive a formal diagnosis and it is often misdiagnosed as depression. Dela Salla's team aims to develop a simple screening test that will help GPs discriminate Alzheimer's from normal ageing and depression.
"An early diagnosis is hugely important as it may enable people with dementia to understand their condition, have access to certain drugs that could help relieve some of their symptoms and enable them to plan for their future long-term care needs.
"One million people will develop dementia in the next 10 years. We must act now."