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Thursday, June 25, 2009

Are 1 in 64 Kids Autistic?

Quite possibly, yes. In the last post I discussed the interesting background to a new paper about the prevalence of autism in British children, Prevalence of autism-spectrum conditions: UK school-based population study. Here's some more about the study itself.

The authors, Simon Baron-Cohen et al from the University of Cambridge, set out to assess the prevalence of “autistic spectrum conditions” in the county of Cambridgeshire, England, by sampling all of the school children aged 5 to 9 years during 2003-2004.

The most recent major study examining the prevalence of autistic spectrum conditions in Britain was Baird et al (2006), who reported a prevalence of about 1 in 86. But Baron-Cohen et al point out that this may have been too low, since Baird only looked for autism in children who were already on the government's “Special Educational Needs (SEN)” register of children with difficulties in school. If there were autistic children who were doing OK in school, or at least well enough to get by without attracting concern, they’d have been missed.

So, Baron-Cohen’s team first wrote to every school in Cambridgeshire (162 of them) and asked them to report how many of their children had been diagnosed with an autism-spectrum condition.
79 schools replied and reported 83 children diagnosed out of 8824 total, or 1 in 106 children – pretty close to Baird et al's in 2006.

But those were only the kids who had already got a diagnosis. In order to try to find undiagnosed cases, they then sent questionnaires to the parents of 11,635 children. The questionnaires included a screening form developed in Cambridge called the CAST, which asks parents about various aspects of their child’s behaviour (“Does s/he come up to you spontaneously for a chat?” “Does s/he like to do things over and over again, in the same way all the time?” Etc.)

The authors invited all of the kids who scored highly on the CAST to a face-to-face assessment to confirm whether they really had the condition. The end result was that out of 3373 kids whose questionnaires were returned, 11 were judged (in the opinion of the research team) to have an autism-spectrum condition which had never been previously diagnosed.

What does this mean? Well, good question. All it strictly means is that 11 out of 3373 children had undiagnosed autism. However, because not all of the children who scored highly on the CAST agreed to be interviewed, the authors estimate that the true figure was probably more like 22. That compares to 33 out of those 3373 whose parents reported already diagnosed autism. (Actually it was 41 reported, but only an estimated 33 were declared “confirmed”. See page 503 if you’re sceptical of this fudge, but it seems kosher to me.)

The bottom line: for every 3 children with a diagnosis, 2 others went undiagnosed. Since about 1 in 100 children have diagnosed autism, that makes 1 in 64 children with autistic spectrum conditions in total.

But this relies on some assumptions. In particular, this only works if you assume that the parents of autistic children were no more or less likely to complete the CAST questionnaire, and no more or less likely to agree to a face-to-face interview, than parents of the non-autistic kids.

However, it could well be that the parents of autistic children were already concerned that there was “something wrong” with their child and wanted to get a professional opinion, so they were keen to take part – that would mean that this study overestimated the rate of undiagnosed autism. On the other hand, it could equally well be that the autistic children were less likely to get included in the study. Maybe they just didn't want to go along to the interview with a stranger. In which case, the rate of autism would be underestimated.

Because only 29% of parents did the questionnaire and even then only about 60% of the children who scored high came up for the face-to-face, the potential for bias is great. Unfortunately, there is no way of knowing which way the bias operates. The authors acknowledge these concerns and admit that their estimates are not exact.

But this is still an important study. Even if you assume that the data were extremely biased towards finding autistic children there were still 11 cases of undiagnosed autism out of about 11,000 kids aged 5-9, compared to 83 diagnosed, which means that at an absolute minimum 1 in 9 children with autism of that age are undiagnosed. And the true figure is likely to be a lot higher, maybe 2 in 5 as the paper claims.

On this blog I've often been skeptical of claims that mental illness is extremely common. But I can easily believe that 1 in 64 children has a significant autism spectrum-condition, and that some cases go undiagnosed in primary school. While we still don't know the exact numbers, and these will always be somewhat arbitrary since they depend upon the chosen diagnostic, about 1 in 50 sounds about right. Certainly, the idea that autism is an extremely rare condition affecting more like 1 in 2000, as was believed twenty years ago, is out of date.

ResearchBlogging.orgBaron-Cohen, S., Scott, F., Allison, C., Williams, J., Bolton, P., Matthews, F., & Brayne, C. (2009). Prevalence of autism-spectrum conditions: UK school-based population study The British Journal of Psychiatry, 194 (6), 500-509 DOI: 10.1192/bjp.bp.108.059345

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