Diagnostic substitution unlikely to explain increases in autism in California

Diagnostic Substitution is one of the factors that have been proposed as a cause, or at least a contributing source, of the dramatic increases in autism observed during the last 10-15 years. The basic idea behind diagnostic substitution is that children that today receive a diagnosis of Autism, would have received a different diagnosis 20 years ago. Thus, the theory would suggest that there has not been an actual increase in the number of children affected, but instead simply a shift in the way conditions are classified.

Evidence for and against diagnostic substitution continue to accumulate. For example, last year I commented on a study showing that some adults that were diagnosed with pragmatic language impairment during their childhood actually meet diagnostic criteria for autism. But in an upcoming issue of the Journal of Autism and Developmental Disabilities, researchers from the California Department of Public Health report the findings of an examination in diagnostic coding practices in California during the past 20 years.

Specifically, the authors examined 4 cohorts of children who were born in 1987, 1990, 1994, and 1997, and who had been seen by the California Department of Developmental Services (DDS). The DDS provides services to mostly children with mental retardation, autism, or both. The authors compared the official diagnostic classification in the DDS system with the actual clinical records and clinical impressions of a representative sample of these children to answer 2 main questions: 1) Was there a coding/diagnostic shift from providing diagnoses of “mental retardation only” to  “mental retardation AND autism”? 2) Was there an increase in the number of children classified as autistic but who did not have supporting evidence for the condition?

The results:

1. In 1987 2.3% of the children classified as having MR only also had clinical evidence of autism but were misclassified as having MR only. In addition 7.4% of these children had evidence of other ASD. However, this proportion of misclassification actually increased (although not statistically significant) during subsequent years. This increase is contrary to what the diagnostic substitution hypothesis would suggest.
2. The authors also found that the proportion of children classified with autism but who did not have clinical evidence of the condition was consistent across all cohort years at less than 12%. In addition, the proportion of children classified as having other ASD but who did not have clinical evidence was under 10%.
3. The authors also found that the number of children classified as having autism with co-morbid mental retardation has decreased. But the number of children with the autism classification and no formal MR evaluation has also increased. Thus, it is possible that the apparent net decrease in kids with dual diagnosis is due to a decrease in the evaluation of MR among kids with autism.

Although not conclusively, these data provide support against the diagnostic substitution hypothesis as an explanation of the observed increases in autism in California. However, as correctly discussed by the authors, the results speak mostly to official ASD diagnostic classifications. For example, the results clearly show there has not been a shift in the number of children who were officially “misclassified” as having MR when they actually had MR and autism. Yet, this does not address possible shifts in actual clinical practices. Specifically, the authors examined discrepancies between the official classification and the clinical records/impressions. Although the authors put significant effort in validating these impressions (for example, examining conflicting impressions, and reviewing these impressions when they were not provided by a qualified professional) there is the underlying assumption that the original clinical impressions were correct if provided by a “qualified professional” and in the absence of conflicting data. Thus, shifts in the accuracy of clinical impressions would not have been captured in this analysis.

Grether, J., Rosen, N., Smith, K., & Croen, L. (2009). Investigation of Shifts in Autism Reporting in the California Department of Developmental Services Journal of Autism and Developmental Disorders DOI: 10.1007/s10803-009-0754-z

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