About 60 percent of the increase in the observed prevalence of autism spectrum disorders (ASDs) in Danish children appears to be largely due to changes in reporting practices, according to a study published online by JAMA Pediatrics. The prevalence of ASDs (neurodevelopmental disorders characterized by impaired social interactions, communication and by repetitive behaviors) has increased over the past 30 years. The current estimate is about 1 percent of children, although it also has been reported to be higher, according to background information in the study. That increase in prevalence has led to debate over how much is due to etiologic factors compared with nonetiologic factors such as changes in reporting practices.

Stefan N. Hansen, M.Sc., of Aarhus University, Denmark, and co-authors quantified the effect of changes in reporting practices in Denmark on reported ASD prevalence. The Danish national health registries have undergone two major changes in reporting practices during the past three decades with a change in diagnostic criteria in 1994 and the inclusion of discharge diagnoses from outpatient contacts in 1995.

The researchers' study included 677,915 children born in Denmark from 1980 through 1991 who were followed up from birth until ASD diagnosis, death, emigration or the end of December 2011, whichever came first.

Study results indicate there were 3,956 ASD diagnoses, with the vast majority of them coming after 1995. About 33 percent of the increase in reported ASD prevalence could be explained by the change in diagnostic criteria alone; 42 percent by the inclusion of outpatient data alone; and 60 percent by the change in diagnostic criteria and the inclusion of outpatient data.

"This study supports the argument that the apparent increase in ASD prevalence in Denmark in recent years is in large part attributable to changes in reporting practices over time. However, a considerable part of the increase in ASD prevalence is not explained by the two changes in reporting practices. Thus, the search for etiologic factors that may explain part of the remaining increase remains important," the study concludes.