Dementia is a progressive disease
Memory and language tests can reliably reveal "hidden" early dementia, say UK experts.
Most dementias are missed for years as the symptoms can be elusive until considerable brain tissue is lost.
But doctors from Oxford found they were able to spot very early warning signs when they looked closely enough.
The findings in Neurology could help doctors diagnose dementia sooner, which is crucial since treatment is most effective when given early.
Over a span of 20 years, the researchers studied a group of 241 healthy elderly volunteers, giving them regular tests designed to measure their thinking or cognitive powers.
When they scrutinised the test results, the doctors found subtle clues that, in retrospect, hinted at ensuing impairment.
Specifically, the patients who went on to develop mild cognitive impairment or pre-dementia stumbled on tasks involving language expression, learning and recall.
For example, they had greater difficulty remembering the name for common objects or animals and explaining the meaning of a given word.
And those who were older and who scored lower on the language or memory tests tended to deteriorate more quickly.
Professor David Smith and his team say their findings fit with what we already know about dementia.
Experts have noted that the early stages of dementia are associated with linguistic problems, such as word-finding difficulties.
Early literary works by authors who have later been diagnosed with Alzheimer's show similar changes in language use - simpler narratives and a smaller vocabulary.
Rebecca Wood of the Alzheimer's Research Trust said: "This significant long-term study shows how subtle, but measurable, problems with language or memory can predict when a healthy elderly person is likely to develop mild cognitive impairment, which frequently develops into dementia.
"Early intervention will be crucial for future dementia treatments. Being able to spot and measure the initial stages of dementia is a crucial challenge if we are to improve drug testing and lay the groundwork for prevention trials."
Latest work in Archives of General Psychiatry adds weight to the evidence that Alzheimer's dementia is at least partly inherited, and that being healthy in mid-life could help lower your risk of the disease.
Dutch researchers found that people with a parental history of Alzheimer's had higher blood pressure and indicators of arterial disease as well as different amounts of inflammatory proteins in their blood compared with those without a parental history of Alzheimer's.