Diagnostic criteria for attention deficit hyperactivity disorder (ADHD) and autism have broadened and are common at estimated adult prevalences of 3%. This paper explores the incidence of autism and overlap of features in ADHD young adults transitioning from Child and Adolescents Mental Health Services (CAMHS) into a specialist adult ADHD service, and the utility of the Ritvo Autism Asperger Diagnostic Scale 14 (RAADS-14) assisting assessment and support planning.

This comparative cohort study included all young adult ADHD patients referred from CAMHS. A comprehensive assessment and diagnostic formulation, including RAADS-14 was completed. Those without a current autism diagnosis but clinical assessment suggested autism, underwent further assessment. Percentage of autistic and ADHD young adults was calculated. RAADS-14 total and subscale scores were compared between groups. Gender differences were assessed.

Co-occurrence of autism in a group of young adults diagnosed with ADHD was high (53%). High levels of autism features were evident in the ADHD only group. Significant differences in the RAADS-14 sub-scores (social anxiety, mentalizing and sensory sensitivities) were found between the autistic ADHD and non autistic ADHD groups. Autistic females scored higher on all domains of the RAADS-14 compared to males. Sensory sensitivities were significantly higher in females in both groups.

The level of co-occurrence of autism, and overlap of features, suggests employment of neurodevelopmental rather than single condition approaches to avoid mis-diagnosis/missed diagnoses. Sensory sensitivities are suggestive of neurodevelopmental differences particularly in females regardless of diagnostic category. The RAADS-14 may be helpful as part of screening and support planning.