Exploring Gender Ratio Time Trends in Australian Children with Autism Spectrum Disorder Using Medicare Data

Background: Many more males are diagnosed with Autism Spectrum Disorder (ASD) than females, with male to female ratios being on average around 4 to 3:1. Some research suggests that milder female cases of ASD have not been identified in the past or are identified later than males. This awareness and the release of the DSM-5 in 2013 acknowledging possible under-diagnosis of females with ASD may have influenced more recent gender ratios. There has also been an increase in ASD prevalence over the last 20 years worldwide. Understanding changes over time in the gender ratio and gender differences in age of diagnosis may inform on various theories relating to why males outnumber females with ASD and the increased prevalence of ASD.

Objectives: This study aimed to use Australian health system Medicare data which captured new ASD diagnoses from 2008-2016 to understand gender trends over time.

Methods: Secondary data analyses from the Australian Medicare system were explored. Two Medicare items that can only be used once when paediatricians/psychiatrists diagnosed a child aged under 13 years with ASD were utilised. Descriptive statistics and regression analyses were used to understand trends over time.

Results: There were a combined total of N=73,463 unique cases identified via the Medicare ASD diagnostic items from 1 July 2008 to 30 June 2016. There were significant increases in new cases of older boys and girls (aged 5-12 years) but not younger children (aged 0-4 years) from 2010/11 through 2015/16. The M:F ratio significantly decreased from 4.1 to 3.0 (p<.001) in this time period in the older children, but the decrease was not statistically significant in younger children (p=.059; 4.2 to 3.5). When using yearly age groups there was a significant decrease in the M:F ratio with increasing age in the years 2014/15 (p=.011) and 2015/16 (p<.001) but no significant decrease in the earlier years (2012/13, 2013/14). Using combined data from 2012/13 to 2015/16, five years of age was the most frequent age of diagnosis for both girls and boys.

Conclusions: Identification of older boys and girls aged 5-12 rather than younger children is contributing to the increased number of ASD cases in Australia. Since 2014 the M:F ratio is decreasing with increasing child age with more older higher functioning females being identified. This could relate to better awareness of milder female cases of ASD following DSM-5 changes which highlight this group. Appropriate services for newly diagnosed primary school aged children should be a focus.