About 6% of school age children have ADHD

A team at London's Brunel University developed the test during a study of 65 four to six-year olds.

Researchers found the test, involving children following a spot of light on a computer screen, proved 93% accurate.

But experts questioned whether one test could diagnose the behavioural disorder when its symptoms varied so much from child to child.

At the moment doctors rely on a number of different techniques, including questionnaires and psychiatric assessments, to diagnose ADHD, but as yet there is no biological test.

I think it would be wonderful if we were to have a test which could accurately diagnose pre-schoolers with ADHD Dr David Coghill

Diagnosis at an early age is hard because many children are often overactive and exuberant.

ADHD affects about 6% of school age but varies in severity.

During the study, children were asked to follow the spot during four different types of movements for 30 seconds across the screen.

The team found that children with ADHD had a different eye movement pattern from children without the disorder.

'Important internationally'

Professor George Pavlidis, one of the researchers, said the test was a real breakthrough as it meant children with ADHD could receive help sooner.

"Early diagnosis of these very important hereditary conditions will allow effective intervention and proper treatment which in turn will reduce the learning, the behavioural and the painful secondary psychological problems of the children."

He said another advantage was that it could be used whatever language a child spoke.

"This biological test proved to be objective and highly accurate and can be used at pre-school age.

"The discovery is also important internationally as the test operates equally effectively regardless of language, race, culture and IQ."

Dr David Coghill, senior lecturer in child and adolescent psychiatry at the University of Dundee, said: "I think it would be wonderful if we were to have a test which could accurately diagnose pre-schoolers with ADHD.

"But it is usual to have a lot of variability with the types of problems children with ADHD have, and that has led us to conclude it's unlikely that we will get one task that can diagnose ADHD.