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Original Investigation
January 2015

Explaining the Increase in the Prevalence of Autism Spectrum Disorders: The Proportion Attributable to Changes in Reporting Practices

Author Affiliations
  • 1Section for Biostatistics, Department of Public Health, Aarhus University, Aarhus, Denmark
  • 2Section for Epidemiology, Department of Public Health, Aarhus University, Aarhus, Denmark
  • 3National Centre for Register-Based Research, Department of Economics and Business, Aarhus University, Aarhus, Denmark
  • 4Lundbeck Foundation Initiative for Integrative Psychiatric Research, iPSYCH, Aarhus, Denmark
JAMA Pediatr. 2015;169(1):56-62. doi:10.1001/jamapediatrics.2014.1893

Importance  The prevalence of autism spectrum disorders (ASDs) has increased markedly in recent decades, which researchers have suggested could be caused in part by nonetiologic factors such as changes in diagnosis reporting practices. To our knowledge, no study has quantified the degree to which changes in reporting practices might explain this increase. Danish national health registries have undergone a change in diagnostic criteria in 1994 and the inclusion of outpatient contacts to health registries in 1995.

Objective  To quantify the effect of changes in reporting practices in Denmark on reported ASD prevalence.

Design, Setting, and Participants  We used a population-based birth cohort approach that includes information on all individuals with permanent residence in Denmark. We assessed all children born alive from January 1, 1980, through December 31, 1991, in Denmark (n = 677 915). The children were followed up from birth until ASD diagnosis, death, emigration, or the end of follow-up on December 31, 2011, whichever occurred first. The analysis uses a stratified Cox proportional hazards regression model with the changes in reporting practices modeled as time-dependent covariates.

Exposures  The change in diagnostic criteria in 1994 and the inclusion of outpatient diagnoses in 1995.

Main Outcomes and Measures  Autism spectrum disorders.

Results  For Danish children born during the study period, 33% (95% CI, 0%-70%) of the increase in reported ASD prevalence could be explained by the change in diagnostic criteria alone; 42% (95% CI, 14%-69%), by the inclusion of outpatient contacts alone; and 60% (95% CI, 33%-87%), by the change in diagnostic criteria and the inclusion of outpatient contacts.

Conclusions and Relevance  Changes in reporting practices can account for most (60%) of the increase in the observed prevalence of ASDs in children born from 1980 through 1991 in Denmark. Hence, the study supports the argument that the apparent increase in ASDs in recent years is in large part attributable to changes in reporting practices.