Friday, 13 November 2015

The autism numbers game: now 1 in 45 (estimated)

I want to (briefly) draw your attention to the findings reported by Benjamin Zablotsky and colleagues [1] (open-access) recently on the topic of the (estimated) autism prevalence rate. Specifically the figure that seems to be making some media headlines: "The estimated ASD [autism spectrum disorder] prevalence was 2.24% (1 in 45) in 2014."

Based on data derived from the 2014 US National Health Interview Survey (NHIS) designed with the purpose "to monitor the health of the United States population through the collection and analysis of data on a broad range of health topics" researchers turned their eyes towards the question of "the lifetime prevalence of ASD, intellectual disability (ID), and any other developmental delay (other DD)." The added interest in this latest version of the NHIS was the inclusion of a "standalone question" to respondents about autism/ASD along the lines of "Did a doctor or health professional ever tell you that [child's name] had autism, Asperger's disorder, pervasive developmental disorder, or autism spectrum disorder?" In previous times, the survey only included autism/ASD among a list of various other health-related conditions including Down syndrome and cerebral palsy.

Taking into account the change in format to the current (2014) survey bringing autism/ASD to the forefront, Zablotsky et al report something of a considerable shift in the number of people reporting a positive response to that question. So based on the estimated prevalence of 2.2% this time around, researchers noted that this was "a significant increase" compared with previous versions of the NHIS (2011-2013) where the figure was 1.25% or an estimated 1 in 80 children (aged 3-17 years old) with autism/ASD. At the same times as an increase in reports of autism/ASD/Asperger syndrome/PDD, there was a significant decrease in the number of reports of 'other developmental delay' (this question being standalone in both the 2011-2013 and 2014 surveys). Reports on the frequency of intellectual (learning) disability were not significantly different between the 2011-2013 and 2014 data (1.2% vs. 1.1% respectively). More discussion about the study can be read here.

The authors (and some of the media) have made quite a bit of the change in questioning format as being a primary variable to account for the estimated prevalence. I would tend to agree that how one asks the question and specifically the prominence of the question, is likely to impact on what responses one gets from these kinds of surveys, also bearing in mind that this was not a 'go out there and objectively screen' kind of autism prevalence study. That being said, I am open to the idea that part of the percentage increase in reports of autism/ASD may also reflect other factors not limited to just something like diagnostic substitution for example.

Whilst the 1 in 45 figure might be a shock to some/many people, I personally am not that surprised. Only this year (2015) I've talked about a figure of 1 in 46 coming out of Canada (see here) making the previous 1 in 68 estimate from the CDC look a little conservative in the context of the North American experience taking into account differences in ascertainment (see here). Irrespective of the discussions around what factors might be contributory to reported autism rates (estimated and actual), such findings suggest that quite a bit more planning and resources may need to be put into catering for the needs of this ever-growing population both in childhood and indeed, beyond.


[1] Zablotsky B. et al. Estimated Prevalence of Autism and Other Developmental Disabilities Following Questionnaire Changes in the 2014 National Health Interview Survey. National Health Statistics Reports. 2015; 87. Nov 13.

---------- Benjamin Zablotsky, Lindsey I. Black, Matthew J. Maenner, Laura A. Schieve, & Stephen J. Blumberg (2015). Estimated Prevalence of Autism and Other Developmental Disabilities Following Questionnaire Changes in the 2014 National Health Interview Survey National Health Statistics Reports