Explaining the Unexplained Growth in Autism Prevalence

Barbara Geller, MD

Autism prevalence is altered in part by changes in diagnostic criteria, inclusion of outpatients, and sex differences.

Rates of autism spectrum disorder (ASD) have been increasing for reasons that are not fully understood. To examine this increase, investigators used data from the Danish Psychiatric Registry.

This registry is considered highly reliable because only child psychiatry specialists can diagnose ASD in Denmark. Also, the investigators were able to examine the effects of two changes in the registry: the change from the 8th edition to the 10th edition of the International Classification of Diseases in 1994 and the addition of outpatient diagnoses in 1995. Participants included birth cohorts between 1980 and 1991 (N=677,915); follow-up continued through December 2011 (3956 participants with ASD diagnoses).

To estimate prevalence changes, the researchers compared rates before and after the two changes to the registry and compared actual rates with those expected from prior trajectories. Overall, 60% of the increase in ASD rates was due to changes in diagnostic criteria or to the inclusion of outpatients, but in subanalyses, the effects due to changes in diagnostic schema were greater in boys.

Citation(s):

Hansen SN et al. Explaining the increase in the prevalence of autism spectrum disorders: The proportion attributable to changes in reporting practices. JAMA Pediatr 2014 Nov 3; [e-pub ahead of print].