Background: There are high rates of comorbidity between ADHD and ASD; however, there has been limited work parsing rates by ADHD presentation. In addition, commonly used questionnaires have demonstrated reduced utility in capturing ADHD symptoms in individuals with ASD. We examined the prevalence of comorbid Attention-Deficit/Hyperactivity Disorder (ADHD) parsed by DSM-5 presentation in clinic-referred youth with Autism Spectrum Disorder (ASD) without intellectual disability (ID). We compared common rating scales to determine which most effectively identified comorbid ADHD. Method: We examined comorbid ADHD diagnoses from archival assessment data for 419 youth with ASD without ID. We examined diagnostic discriminability of the parent and teacher ADHD Rating Scale (ADHD R-S), and Attention and ADH Problems Scales of the Child Behavior Checklist and Teacher Report Form using receiver operating characteristic (ROC) curves. Hierarchical logistic regression was used to examine measures’ unique contribution to ADHD diagnosis. Results: Sixty-one percent of the study sample met DSM-5 criteria for an attention disorder. ADHD, Combined (ADHD-C) represented the largest proportion of ADHD diagnoses (76.8%), followed by Inattentive (ADHD-I;19.7%), Hyperactive/Impulsive (.02%), and Un-/Other Specified (.02%). Measures provided greater diagnostic discriminability in identifying ADHD-C relative to ADHD-I. The ADHD R-S inattentive symptom count provided the greatest discriminability for both subtypes and was the only scale that provided clinically meaningful differentiation between those with ASD only and ASD + ADHD-I. Conclusions: These results support using the ADHD R-S to capture comorbid ADHD symptoms in ASD. The findings underscore the need for more thorough examination of inattentive symptoms to rule out ADHD-I.

Introduction

Attention-Deficit/ Hyperactivity Disorder (ADHD) is one of the most common comorbid disorders diagnosed in children with Autism Spectrum Disorder (ASD) with rates of co-occurrence ranging from 30 to 70% (Antshel, Zhang-James, Wagner, Ledesma, & Faraone, 2016; Joshi et al., 2017; Lee & Ousley, 2006; Leitner, 2014; Leyfer et al., 2006; Simonoff et al., 2008; Sinzig, Walter, & Doepfner, 2009; Taurines et al., 2012). Joshi et al. (2017) report that the clinical presentation of ADHD in youth with ASD is similar to its presentation outside the context of ASD with respect to age of onset, distribution of diagnostic presentations, symptom profile, and symptom severity. In their psychiatrically referred sample of 140 youth with ASD without intellectual disability (ID), 76% also met diagnostic criteria for ADHD per the Diagnostic and Statistical Manual of Mental Disorders (4th ed.; DSM-IV; American Psychiatric Association, 1994). Moreover, 41% of youth with ASD with comorbid ADHD had not been identified as having an attention disorder until their participation in the study, and thereby were less likely to have received appropriate treatment. Evidence suggests that individuals with comorbid ASD and ADHD benefit from pharmacological interventions that address symptoms of ADHD (Santosh, Baird, Pityaratstian, Tavare, & Gringras, 2006; Taurines et al., 2012); thus, failing to appropriately identify a comorbid ADHD diagnosis may preclude these children from receiving evidence-based treatments and contribute to greater adaptive and/or behavioral challenges (Jang et al., 2013; Joshi et al., 2017; Posserud, Hysing, Helland, Gillberg, & Lundervold, 2018; Yerys et al., 2009). In fact, the growing awareness of, and evidence for, the presence of functionally impairing co-occurring ADHD symptoms in youth with ASD led to the removal of the exclusivity clause between the diagnoses with the most recent iteration of the DSM (5th ed.; DSM-5; American Psychiatric ssociation, 2013; Antshel et al., 2016; Colombi & Ghaziuddin, 2017; Goldstein & Schwebach, 2004; Russell, Rodgers, & Ford, 2013; Sprenger et al., 2013; Yoshida & Uchiyama, 2004). The majority of studies examining ASD and ADHD comorbidity do not parse the different presentations of ADHD, and existing studies were completed prior to the publication of DSM-5 (2013).