Prevalence Counts: Commentary on "Prevalence and Characteristics of Autism Spectrum Disorder Among 4-Year-Old Children in the Autism and Developmental Disabilities Monitoring Network"

T he biennial release of the prevalence of autism spectrum disorder (ASD) in 8-year-old children by the Autism and Developmental Disabilities Monitoring (ADDM) network of the Centers for Disease Control1 plays a critical public health role by estimating the number of children with ASD in the United States. Prevalence reports are used by scientists to compare findings in samples ascertained from the community, by funding agencies such as the National Institutes of Health to prioritize funding decisions, by policy makers to address the needs of children with ASD, and by many others in the community. The article by Christensen in the current issue of JDBP2 used the same methods in a subset of the ADDM sites to determine the prevalence of ASD in 4-year-old children. The key finding is that within the specified catchment areas, the prevalence of ASD in 4-year-old children is 30% lower than the prevalence of 8-year-old children. This finding points to a gap between current knowledge of early manifestations of ASD and our ability to use such knowledge to detect and record ASD symptoms in preschoolers. When considering differences in prevalence at age 4 and age 8 in the context of record-review surveillance methodology, 2 distinct questions must be considered: (1) is all ASD detectable at early ages, meaning are symptoms at clinically significant levels at the time of assessment? (2) Do records adequately capture all cases that have detectable ASD symptoms? Regarding the first question about when ASD symptoms reach clinically significant levels, it is evident from