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The prevalence of autism spectrum disorders: impact of diagnostic instrument and non-response bias

Centre for Child and Adolescent Mental Health, Unifob Health, Jonas Lies vei 91, 5009, Bergen, Norway.
Social Psychiatry (Impact Factor: 2.58). 07/2009; 45(3):319-27. DOI: 10.1007/s00127-009-0087-4
Source: PubMed

ABSTRACT A large part of the variability in rates of autism spectrum disorders (ASD) across studies is non-aetiologic, and can be explained by differences in diagnostic criteria, case-finding method, and other issues of study design.
To investigate the effects on ASD prevalence of two methodological issues; non-response bias and case ascertainment. We compared the findings of using a semi-structured parent interview versus in-depth clinical assessment, including an ASD specific interview. We further explored whether including information on non-responders affected the ASD prevalence estimate.
A total population of 7- to 9-year olds (N = 9,430) was screened for ASD with the autism spectrum screening questionnaire (ASSQ) in the Bergen Child Study (BCS). Children scoring above the 98th percentile on parent and/or teacher ASSQ were invited to participate in the second and subsequently in the third phase of the BCS where they were assessed for ASD using the Development and Well-Being Assessment (DAWBA), and the Diagnostic Interview for Social and Communication disorders (DISCO), respectively.
Clinical assessment using DISCO confirmed all DAWBA ASD cases, but also diagnosed additional cases. DISCO-generated minimum prevalence for ASD was 0.21%, whereas estimated prevalence was 0.72%, increasing to 0.87% when adjusting for non-responders. The DAWBA estimate for the same population was 0.44%.
Large variances in prevalence rates across studies can be explained by methodological differences. Both information about assessment method and non-response are crucial when interpreting prevalence rates of ASD.

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    • "The concept of autism has evolved from the description of severe cases of infantile autism affecting about 0.02% (Kanner, 1943), to the modern day autism spectrum disorder (ASD) encompassing an estimated 1% of the population (Baird et al., 2006; Brugha et al., 2011; Posserud, Lundervold, Lie, & Gillberg, 2010). Needless to say, the ''1% ASD'' is not the same as ''0.02% infantile autism''. "
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    • "The prevalence of ASD has dramatically increased to as high as approximately 9 and 11.3 per 1,000 in the US in most recent surveys in 2006 [report, 2009] and 2008 [report, 2012], respectively, and males are more often affected than females with a ratio of 4:1 [Kogan et al., 2009; Brugha et al., 2011; Chien et al., 2011b; Kim et al., 2011]. There is a large variation in the prevalence rates between studies [Brugha et al., 2011; Kim et al., 2011] explained by varied diagnostic and methodological approaches employed as well as different levels of awareness about ASD [Posserud et al., 2010]. Due to its high prevalence, long-term impairment, high genetic component, and lack of effective prevention and treatment, ASD has been prioritized for genetic studies [Merikangas and Risch, 2003]. "
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    • "In addition, diagnostic substitution from mental retardation to autism has been suggested to account for as much as 25 % of the increase (King and Bearman 2009). Other possible explanations for the increases include greater ASD awareness (Posserud et al. 2010), earlier age of diagnosis, changes in diagnostic criteria (Fombonne et al. 2011) and methodological differences (Wazana et al. 2007). Despite a general recognition that broader diagnostic criteria have influenced changes in prevalence, changes in diagnostic criteria and younger age at diagnosis were felt to explain only a fraction of the dramatic rise in autism diagnoses in California (Hertz- Picciotto and Delwiche 2009). "
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