Comparing Performance of Children with ASD and ADHD on the Autism Diagnostic Observation Schedule

Background: Autism Spectrum Disorder (ASD) and Attention Deficit-Hyperactivity Disorder (ADHD) are prevalent developmental disorders with significant symptom overlap.  Among school age children/adolescents with fluent language, differentiating social deficits associated with ASD from those of ADHD may be difficult.  For example, ASD symptom measures such as the Social Responsiveness Scale and the Social Communication Questionnaire have been shown to have reduced specificity in samples of children with ADHD (e.g., Reiersen et al., 2007; Charman et al., 2001). 

Assessment of ASD often involves administration of the Autism Diagnostic Observation Schedule (ADOS), a semi-structured diagnostic instrument that provides a classification of Autism, ASD or non-spectrum. The ADOS is one of the most well-established measures for assessment of ASD. However, as with other ASD measures, children with certain non-ASD diagnoses may be likely to receive falsely elevated scores on the ADOS (e.g., C. Molloy et al 2010; Bishop et al., 2007)

Objectives: To assess the utility of the ADOS scoring algorithm in differentiating ASD referrals who received a clinical diagnosis of ASD from those who received a clinical diagnosis of ADHD, and to identify the most discriminating items.

Methods: Records were reviewed for all children who underwent a multidisciplinary clinical evaluation for ASD (including ADOS) at Cincinnati Children’s Hospital Medical Center in 2009. The subset of children evaluated with a Module 3 (Fluent Speech) was examined. The group with a final diagnosis of ASD was compared to the group given a final diagnosis of ADHD on demographics, measures of behavior and cognition and ADOS total and domain scores.  Sensitivity and specificity was calculated. Item scores were dichotomized to 0 (no abnormality) and non-0 (at least some abnormality). The proportion of children per group with some evidence of abnormality was compared for each item.

Results: 504 children were evaluated for ASD in 2009. Of the 164 assessed with a Module 3, 52 were diagnosed with ASD, 44 with ADHD. Groups did not differ on age, race, sex or Full Scale IQ. The ADOS had a .77 sensitivity and .44 specificity for differentiating ASD from ADHD in this sample. Median total scores for both groups were above ASD cut-offs. The communication domain score was significantly higher in the ASD group (p = 0.01). Overall, six items (two on the algorithm) showed significant differences.

Conclusions: These results underscore the importance of considering information from the ADOS together with other sources when making clinical diagnoses of ASD or ADHD.  Children with ADHD who are referred for ASD evaluations may be particularly likely to receive elevated scores on ASD measures. Given recent referral trends in ASD clinics (e.g., to "rule out" ASD in children with higher cognitive and language abilities), future revisions to ASD diagnostic instruments may benefit from the inclusion of additional items better able to differentiate ASD from ADHD in verbally fluent children.  More research directly comparing ASD and ADHD is needed to determine which symptoms and behaviors have the highest levels of ASD specificity.