Gender Differences in the Age at Diagnosis of ASD

Background:  

Autism spectrum disorders (ASD) are more common in boys than girls. Several studies have shown gender differences in cognitive and clinical manifestations of ASD (Holtman et al., 2007) and in the prevalence of co-existing conditions (Hartley & Sikora, 2009) and motor difficulties (Carter et al., 2007). Girls are more likely than boys to have received a diagnosis if they also present with a learning disability than if they just present with ASD alone (Mandy et al., 2012; Van Wijngaarden-Cremers et al., 2013). Recently, there has been a growing awareness of ASD in girls, and a recognition that under-diagnosis is common.

Objectives:

To investigate gender differences in the age at ASD diagnosis: i) whether girls’ age at diagnosis has reduced, compared to boys’, across two age cohorts; ii) whether age at diagnosis differed between girls and boys diagnosed aged < 60 months or ≥ 60 months; and iii) whether certain ASD characteristics were associated with earlier diagnosis in girls.

Methods:

Data were available from two large representative UK databases: The Database of Children with Autism Spectrum Disorder Living in the North East (Daslⁿe) and the Autism Spectrum Database-UK (ASD-UK; Warnell et al., 2015). Age at diagnosis was first examined for 641 boys and 111 girls enrolled in Daslⁿe by Year of Birth across two time periods (1996-1999 and 2002-2005). Age at diagnosis was then examined for 2573 boys and 541 girls by Age at Diagnosis Group (< 60 months and ≥ 60 months) from Daslⁿe and ASD-UK, and the correlates between ASD characteristics, gender and age at diagnosis were examined (ASD severity measured by Social Communication Questionnaire (SCQ); learning (intellectual) disability; DSM-IV ASD diagnosis). 

Results:

There was no overall significant gender difference in age at diagnosis comparing Year of Birth age cohorts (1996-1999: boys, median =58.4 months vs. girls, median =81.9 months; 2002-2005: boys, median =54.4 months vs. girls, median =62.6 months, F(1, 748)=1.64, p=.20). However, age at diagnosis for children who received their diagnosis aged ≥ 60 months was significantly lower for boys (median =90.8 months) than for girls (median =101.4 months) (t(340)=-4.00, p<.001). There was no difference between the ASD severity of girls and boys who were diagnosed aged < 60 months (t(803)=.20, p=.84) or those aged ≥ 60 months (t(615)=1.15, p=.25) (data from ASD-UK only). Girls who received their diagnosis aged ≥ 60 months were more likely than boys to have a learning (intellectual) disability (X²(N=1358)=13.33, p<.001) and a diagnosis of autism than another type of ASD diagnosis (X²(N=1358)=12.15, p<.001).

Conclusions:

Girls diagnosed with ASD at 5 years or older were, on average, diagnosed later than boys. Their age at diagnosis did not decrease significantly over the time period considered. Girls in this older group did differ in some ASD characteristics when compared with boys. Clinicians and researchers need to understand the qualitative differences in autism characteristics in girls and boys, how these differences impact on age at diagnosis, and whether specific characteristics can be used to identify girls with ASD at an earlier age.