A national survey of parents published in today’s issue of Pediatrics suggests that the prevalence of autism spectrum disorders (ASDs) is 1 in 91 U.S. children. The study, “The Prevalence of Parent-Reported Diagnosis of Autism Spectrum Disorder Among Children in the United States, 2007,” draws on data from a survey conducted jointly by the Health Resources and Services Administration and the Centers for Disease Control and Prevention.

I expect this study will get a little press.

How is this report explained?

Until now the prevalence of autism had been estimated at 1 in 150 based on an ADDM study from 2002. The authors of this new study suggest the increased prevalence might be partly explained by differences in the ways the surveys were conducted. The current inclusion of milder forms of autism, pervasive developmental disorder and Asperger syndrome could be increasing the number of children diagnosed with ASD.

Keep in mind that this study is based on parent reports of diagnoses that were given to their children by community physicians. It may not be as accurate as a study based on data validated by record review or direct testing. It may not be the accuracy of parents, but the accuracy of clinicians in an atmosphere of demand for a diagnosis.

Increasing autism prevalence may be an example of “diagnostic substitution.” Changes in the way the diagnostic criteria are applied contribute to diagnostic substitution, whereby children who would have been given a different diagnosis (e.g. mental retardation or language disorder) in the past are now diagnosed with an ASD.

And this is important: Previous studies have shown the average age of diagnosis is decreasing, which leads to an increase in total prevalence at any one point in time. While we have many estimates of prevalence, we do not yet know or have accurate estimates of rates of incidence (onset of new cases).

It must be the vaccines

Assertions that toxic effects of the measles-mumps-rubella (MMR) vaccine or a combination of thimerosal-containing vaccines are responsible for the increase in prevalence of ASDs have been incontrovertibly refuted by an Everest-sized pile of epidemiological studies originating from different countries throughout the world. The weight of all of the scientific evidence overwhelmingly favors rejection of the hypothesis that there is a causal association between vaccines and ASDs.

In countries with different patterns and timing of immunizations, the rates of ASD are not different than in the U.S. Further, there’s no evidence that changes in the vaccine schedule or the components of individual vaccines are causing a higher prevalence of autism.

Why did some children “lose” their autism diagnosis?

This was an interesting finding in the study: About 38 percent of the children who were ever diagnosed with ASD were no longer reported by their parents to have diagnosis.

But diagnosing ASD in very young children can be tricky. Some of these children may no longer meet the criteria for ASD as they grow. ASD may have been initially suspected on the basis of a developmental screening, but subsequently ruled out and never diagnosed.

And unfortunately, some children with developmental delay, intellectual disability, and learning disabilities may have been initially classified as having ASD to help them get publically funded services. Of interest, children who had “lost” their ASD diagnosis were more likely than children without ASD to be diagnosed with other developmental or mental health conditions, such as ADHD, anxiety problems, or behavioral problems.

As pediatricians we have work to do. While comparison between the 2002 ADDM study and the current CDC study illustrates the precarious nature of autism epidemiology, it appears that ASDs are to some extent more common than they used to be. But why and at what rate remains to be seen. The Kogan study will serve as an important point of reference in our slow march towards understanding autism.