A five-minute iPad test did not detect dementia until years before the onset of symptoms.
The participants are asked which of 100 pictures on the screen contain an animal.
Some of them are obvious while others are harder to recognize. Experts say earlier diagnosis can lead to more effective treatment.
And the test, which can be taken at home without medical supervision, could also reduce the number of "worried" people sent for scans.
The first picture contains no animal, the second and the third picture.
It is being tested by South London and the Maudsley NHS Foundation Trust. It will be used in North East London, Sussex and Devon next month, with a national rollout planned next year.
A prostitute who earns £ 2,000 a week shows the average penis size … and how much she enjoys working
NOT ONLY SPOTS!
Six common types of bumps on the skin – and those that could be fatal
The truth about your double chin, what it really causes – and the DAFT exercises that can help banish it
What causes an ulcer in the mouth and how are these treated at home?
Red, White & Feeling Blue: Millions of Britons on "Happy Pills"
It was developed by medical device company Cognetivity.
CEO Sina Habibi said: "Every picture is different in terms of complexity."