There may be many children with undiagnosed autism
A significant number of children with autism and related disorders could be undiagnosed, a study has suggested.
A Cambridge University team looked at existing diagnoses - and carried out recognised tests to assess other children.
Of the 20,000 studied, 1% had an autistic spectrum disorder, 12 times higher than the rate 30 years ago.
Autism experts said it was crucial to have accurate data on how many children were affected by the disorder.
The research, published in the British Journal of Psychiatry, was carried out in three parts.
The scientists first looked at cases of autism and Asperger syndrome among 8,824 children on the Special Educational Needs registers in 79 schools in East Anglia.
A total of 83 cases were reported, giving a prevalence of 94 in 10,000, or 1 in 106 children.
The team then sent a diagnosis survey to parents of 11,700 children in the area.
From 3,373 completed surveys, 41 cases of autism-spectrum conditions were reported, corresponding to prevalence of 1 in 101.
This 1% rate confirms estimates from previous research.
They then sent the Childhood Autism Screening Test (CAST) to the same parents to help identify any undiagnosed cases of autism-spectrum conditions.
All those with high scores, plus some who had medium and low scores, were called in for further assessment.
The team found an additional 11 children who met the criteria for an autism spectrum condition, but had not yet been diagnosed.
The researchers say that, if these findings were extrapolated to the wider population, for every three known cases of autism spectrum, there may be a further two cases that are undiagnosed.
Professor Baron-Cohen said: "In terms of providing services, if we want to be prepared for the maximum numbers that might come through, these undiagnosed cases might be significant.
"It is important to conduct epidemiological studies of autism spectrum conditions so that the relevant services, including education, health and social services, can plan adequate provision for all those children and adults who may need support."
Mark Lever, National Autistic Society chief executive, said: "This is important research, which for the first time gives us an estimate of the number of people who don't have an autism diagnosis but may be in need of support.
"Getting the right support at the right time is vitally important and access to appropriate diagnostic services is crucial."
He said the NAS was campaigning for statutory guidance for diagnosis included as part of the proposed Autism Bill to try and improve improvement in local authority and NHS services.