Making use of the robust data in their national health registries, the Danish researchers analyzed the prevalence of autism among all children born in the country from 1980 to 1991. This encompasses a period right before two significant changes in physician reporting: the switch to the 10th revision of the International Statistical Classification of Diseases (ICD-10) in 1994 and the inclusion of psychiatric diagnoses for outpatient admissions in 1995.
They modeled different scenarios based on reported diagnoses before, during, and after these changes and concluded that these two changes explained 60 percent of the observed rise in ASD; ICD-10 revisions alone explained about 33 percent of the rise, while the outpatient effect explained 42 percent.
The authors also found that the ICD-10 changes had a much larger effect on ASD diagnoses in males compared with females, whereas outpatient effect did not show large gender differences.
To learn more about some recent autism prevalence data, see the Psychiatric News article “Prevalence Rate of Autism Continues Steady Rise.”