Attention-Deficit/Hyperactivity Disorder (ADHD) affects an estimated 5.6% of adolescents and 7.6% of children worldwide (Salari et al., 2023). Moreover, approximately two-thirds of children with ADHD also have at least one co-occurring condition (Danielson et al., 2018; Reale et al., 2017). Considering co-occurring conditions and undertaking differential diagnosis is crucial when diagnosing ADHD, due to the longer-term impacts these may have on a child. However, although symptoms of ADHD can be relatively straightforward to recognise (Parker & Corkum, 2016), in clinical practice, the diagnostic process is often complicated, as symptoms of ADHD can mask co-occurring conditions and co-occurring conditions may mask ADHD symptoms (Young et al., 2020). While clinical guidelines support ADHD assessment, existing guidelines offer limited clarity regarding differential and co-occurring diagnoses (Power et al., 2022). This study aimed to develop supplementary guidance for ADHD assessment in complex cases by establishing consensus-based best-practice recommendations for co-occurring and differential diagnosis. A three-round e-Delphi study was conducted with 39 psychologists from Australia, New Zealand, Canada, and the USA, following ethics approval from the Monash University Human Research Ethics Committee. Participants identified co-occurring conditions they had experience assessing. In the first two rounds, statements that reached 75% agreement were considered to have achieved consensus; the final round was conducted for review and feedback on best-practice recommendations. Consensus was achieved on best-practice recommendations in the following areas: intellectual disability, autism, specific learning disorders, anxiety disorders, mood disorders, and trauma-related disorders. Recommendations could not be developed for Tourette’s disorder, developmental coordination disorder, oppositional defiant disorder, stereotypic movement disorder, and reactive attachment disorder due to limited participant experience of these conditions. These findings inform the development of supplementary guidance for ADHD assessment in children, particularly regarding differential diagnosis and co-occurring conditions. The consensus-based recommendations may support psychologists with the assessment of co-occurring and differential diagnoses, and potentially create more consistency among clinicians. Furthermore, the lack of consensus and limited practitioner experience with certain disorders highlights the need for further research to address gaps in understanding and clinical practice variability.

- Gain insight into current consensus-based best-practice recommendations for identifying ADHD alongside common co-occurring conditions (e.g., autism, specific learning disorders, trauma-related disorders).
- Learn consensus-based strategies for differential diagnosis, including specific assessment tools and multi-informant approaches.
- Recognise areas of ambiguity and lack of consensus identified within this study, and appreciate the importance of individualised, evidence-informed clinical decision-making.
- Reflect on the implications of accurate versus missed or misdiagnosed ADHD and co-occurring conditions for treatment planning, intervention, and long-term outcomes for children and families.