Understanding how changes in diagnostic criteria impact identification has important implications for our understanding of ASD trends. Changes in diagnostic criteria are widely understood as a key driving factor behind increased ASD prevalence. However, there have been relatively few empirical studies of this issue. Looking back at earlier work that identified people with an ASD and putting it into a current context can help us understand the extent to which historical work remains relevant today. In the mid-1980’s, researchers at the University of Utah and University of California – Los Angeles (UCLA) collaborated to conduct an autism epidemiology study in Utah. People were identified as an ASD case using DSM-III criteria and were ascertained through queries to the public, prior study participants, providers, group homes, and schools. The oft-reported prevalence of autism was 4 per 10,000.
Objectives:
This study re-examined diagnostic data from a state-wide Utah epidemiological study of autism conducted in the 1980’s with the current DSM-IV-TR case definition and record review methods of Autism and Developmental Disabilities Monitoring (ADDM) Network prevalence studies. The purpose of this study was to examine differences in classifying autism based on individuals identified before increased autism awareness or the inclusion of autism as a special education classification starting in the early 1990’s.
Methods:
Records were reviewed from 241 participants who were between the ages of 3 and 25 years and were classified as “Diagnosed Autistic” in the 1980’s according to DSM-III criteria, and also from 108 who were evaluated and classified as “Not Autistic” according to DSM-III criteria. We applied the records review method and DSM-IV-TR ASD case definition utilized by the Centers for Disease Control and Prevention’s Autism and Developmental Disabilities Monitoring (ADDM) Network.
Results:
Of the 108 records re-reviewed in the original “Not Autistic” group, 64 individuals (59%) met the current case definition of ASD. This represents a significant increase in the percentage of ascertained individuals who met the current case definition for ASD (N=305) compared to the 1980’s case definition (N=241) (z=3.93, p<.0001). Contrary to our expectations, however, the average IQ estimate in the reclassified as autistic group (IQ=35.58; SD=23.01) was significantly lower than in the original group (IQ=56.19 SD=21.21; t=5.75; p<.0001).
Conclusions:
The original sample identified through active case ascertainment in the 1980’s would meet current DSM-IV diagnostic criteria, suggesting that the phenotyping contribution of this early work remains valid. If today’s diagnostic criteria had been applied to cases ascertained in the 1980’s, even more cases of lower-functioning autism would have been identified. Since the sample was based on people identified in the 1980’s, it is likely that additional individuals with high-functioning and milder forms of autism were not even brought to the attention of autism clinicians or researchers in the 1980’s and not included in prevalence estimates.
The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the CDC.
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