Autism Diagnostic Substitution

There is debate about the increase in autism cases being caused by a diagnostic substitution or diagnostic shift from the mental retardation category to the autism category.

This theory is supported by a paper published in the journal Pediatrics in 2006 (Shattuck PT), where the author illustrates that as the autism prevalence increased, the prevalence of mental retardation decreased by an approximately equal amount. However, the paper does not directly analyze the possibility of diagnostic substitution between mental retardation and developmental delay. The developmental delay category was introduced in 1997.

The graph above directly examines the possibility of a diagnostic substitution between mental retardation, developmental delay and autism. The graph illustrates 3 key points:

• The prevalence of mental retardation does not start to decline until the introduction of the developmental delay category. The developmental delay category was introduced the 1997 school year, which is the 1989 birth cohort on the graph above. For school years 1993 to 1997 there is no decline in mental retardation, while the autism prevalence increases.

• A diagnostic substitution between mental retardation and developmental delay fits the data better than a diagnostic substitution between mental retardation and autism. If you assume a diagnostic substitution from mental retardation to developmental delay the combined prevalence does not show a significant change from 1993 to 2006 birth cohorts. During the same time period there is a significant increase of autism prevalence.

• The decline in mental retardation prevalence from years 1993 to 2007 only accounts for 68% of the increase in autism prevalence during the same time period. Autism prevalence increased from 5.48 to 62.31 per 10,000 births, while mental retardation declined from 101.13 to 62.62. Therefore, if there was a diagnostic substitution from mental retardation to autism it could not fully explain the increase in autism prevalence.

References:
Shattuck PT. The contribution of diagnostic substitution to the growing administrative prevalence of autism in U.S. special education. Pediatrics. 2006;117:1028-1037.