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Autism increase in Denmark linked to reporting methods

Changes to diagnostic criteria explained the apparent increase in prevalence of autism spectrum disorders in Denmark, according to recently published research.

Researchers analyzed data from a population-based birth cohort of children born from January 1980 through December 1991 in Denmark. The 677,915 children were followed from birth until diagnosis with ASD, death, emigration or the end of the follow-up period on Dec. 31, 2011. During that time, diagnostic criteria changed in 1994 with the introduction of ICD-10 codes, which are more inclusive of the entire spectrum of related disorders, and in 1995 when outpatient diagnoses were included in the data.

Among the cohort, 3,956 children received a diagnosis of ASD. Of those, 192 were reported before 1994, and an additional 100 from 1994 through 1995; 3,664 cases were reported after 1995.

Statistical analysis showed that the diagnostic change effect was estimated as an HR of 1.42 (95% CI, 0.99-2.04). The effect of outpatient reporting was estimated as an HR of 1.62 (95% CI, 1.24-2.12). The effect was statistically significant in males but not in females.

“The study demonstrated that most (60%) of the increase in ASD prevalence in children born from 1980 through 1991 in Denmark based on registry-reported diagnoses can be explained by the change in diagnostic criteria in 1994 and the inclusion of outpatient data to the Danish Psychiatric Registry in 1995,” the researchers wrote, and noted other effects may be responsible.

“The diagnostic change might have led to a gradual increase in prevalence caused by, for example, a growing awareness among clinicians of the ASD features after the change that could have gradually altered the clinical perception of ASD.”