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J Autism Dev Disord. 2009 Mar;39(3):464-70. doi: 10.1007/s10803-008-0645-8. Epub 2008 Sep 13.

The incidence of clinically diagnosed versus research-identified autism in Olmsted County, Minnesota, 1976-1997: results from a retrospective, population-based study.

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  • 1Department of Pediatric and Adolescent Medicine, Division of Developmental and Behavioral Pediatrics, College of Medicine, Mayo Clinic, 200 First Street SW, Rochester, MN 55905, USA.


Autism prevalence studies have often relied on administrative prevalence or clinical diagnosis as case-identification strategies. We report the incidence of clinical diagnoses of autism spectrum disorders (ASD), versus research-identified autism among residents of Olmsted County, Minnesota, age < or =21 years, from 1976-1997. The incidence of clinically diagnosed ASD (with 95% CI) was 1.5 per 100,000 (0.0-3.7) in 1980-1983 and 33.1 (22.8-43.3) in 1995-1997, a 22.1-fold increase. In contrast, the incidence of research-identified autism increased from 5.5 (1.4-9.5) per 100,000 to 44.9 (32.9-56.9), an 8.2-fold increase. Only 46.8% of research-identified cases received a clinical diagnosis of ASD. These findings demonstrate the potential for misleading interpretation of results from epidemiologic studies that rely on clinical diagnosis of autism to identify cases.

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