As I read Autism related blogs, discussion forums, and news articles, I see many people using the number 1 in 150 to refer to the current rates of autism. Yet, few know or understand where this number comes from and how it was obtained. I hope this brief review can provide some clarity on this issue:
The autism rate of 1 in 150 was published by the US Center for Disease Control in 2007 as part of a weekly disease morbidity and mortality surveillance report. The 1 in 150 rate was obtained from a population-based study of 8 year old children conducted in 2002. Specifically, teams in 15 US States reviewed health and educational records of children born in 1994. Trained clinicians classified them as having an autism spectrum disorder if:
1) had a documented previous classification of an ASD (i.e., the child had either an uncontradicted record of an autistic disorder or ASD diagnosis provided by a qualified examiner or documentation of qualification for special education services during 1994–2002 under an autism eligibility category)
2) did not have a documented ASD classification but had an evaluation record from an educational or clinical source indicating unusual social behaviors consistent with an ASD.
However, the clinical team conducted an additional detailed analysis of the records to ensure that the accepted DSM-IV criteria for autism and ASD was met prior to classifying each child. Thus, classification was not only based on prior records, but also included a secondary analysis by a clinical team that utilized structured procedures to maximize the validity and reliability of their diagnostic process.
Overall, the teams reported a rate of 6.6 per 1,000 children as meeting the diagnostic criteria for ASD (1 in 151). Rates by State varied significantly, but this was affected by differences in the way rates were obtained. Some States were able to determine rates based on health records AND educational records, while others could not get access to educational records. As expected, States that had access to educational records had higher prevalence rates than those that only examined health records. On average, States with access to educational and health records reported an autism prevalence rate of 7.2 per 1,000 (1 in 139), while those with only access to health record reported a rate of 5.1 per 1,000 (1 in 196). The male to female ratio significantly varied by State and ranged from 3.4 to 1 in Maryland to 6.5 to 1 in Utah.
Things to keep in mind:
- This report was based ONLY on children born in 1994. Thus it is possible that the rates could not apply to other cohorts.
- The differences in prevalence rates between States with and without access to educational records could suggest that 1) the overall rate is an underestimate because some sites only had access to health records, or 2) that the overall rate is an overestimate because some sites included cases ascertained from educational records which may be less reliable than health records.
- This rate of 1 in 150 does not refer to new cases of autism, or total cases in the population. It only speaks to cases among 8 year old children in 2002.
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