Autism prevalence among a population of school children in South Korea aged 7 to 12 years was 2.64%, much higher than the 0.7% to 1.8% previously estimated in Western studies, suggesting that variability in screening methods may influence outcomes.

“[R]igorous screening and comprehensive population coverage are necessary to produce more accurate ASD prevalence estimates and underscore the need for better detection, assessment and services,” Young Shin Kim, MD, PhD, of the Child Study Center at Yale University School of Medicine, in New Haven, Conn., and colleagues wrote in the American Journal of Psychiatry.

The researchers screened children at 33 schools in the Ilsan district of Goyang City South Korea that had not yet calculated autism spectrum disorder (ASDs) prevalence. Children were divided into two groups: a general population group (n= 36,592) and a disability group (n=294).

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Researchers distributed the Autism Spectrum Screening Questionnaire [ASSQ] to parents and teachers and performed confirmatory diagnostic evaluation for children who screened positive. A total of 23,234 children in the normal group and 103 children in the disability group completed surveys.

A total of 1,214 children screened positive for an ASD, 286 of who completed diagnostic follow-up and 201 of whom were confirmed as having an ASD for a prevalence rate of 0.36%. However, prevalence jumped to 2.64% after the researchers adjusted estimates to account for children who did not participate.

Children in the general population group accounted for 1.89% of the estimate, whereas children in the high-risk group accounted for the remaining 0.75% — a “striking finding that many of the children in our study were in regular schools, without having been diagnosed and without support,” according to the researchers, who added that the findings highlight “the importance of methodology in determining prevalence rates.”

However, the researchers warned that the overall low response rate (63%) may have biased the sample, with parents of children with an ASD accounting for a greater proportion of the study population and parents of children without an ASD less likely to participate.

“It could be argued that in this cultural context, with few services for children with developmental disorders, parents of children with serious but unrecognized developmental problems would be more likely to participate in research than parents of unaffected children,” the researchers wrote.

Other study findings included lower male-to-female ASD ratios than previously reported – 2.5-to-1 in the general population and 5-to-1 in the disability sample. Furthermore, there were more children with higher IQ scores in the general sample vs. the special education sample (12% versus 7%) and more children with intellectual disabilities in the special education sample vs. the general sample (59% versus 16%).

Differences between the two groups “may be crucial for understanding the full range of autism spectrum characteristics and for proper phenotyping in etiologic and treatment studies,” the researchers wrote, noting that more studies are needed to replicate these findings.

Kim YS. Am J Psych 2011; DOI:10.1176/appi.ajp.2011.10101532.