Investigating reasons behind a delayed or missed diagnosis of Attention Deficit Hyperactivity Disorder in young people, and sex differences

Barclay, Isabella Rose 2025. Investigating reasons behind a delayed or missed diagnosis of Attention Deficit Hyperactivity Disorder in young people, and sex differences. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Attention Deficit Hyperactivity Disorder (ADHD) is a common neurodevelopmental condition. Timely diagnosis and treatment mitigates impact and optimises outcomes. Females are more likely to receive a diagnosis later, or not at all. My thesis aimed to better understand why some young people have a later, or missed, ADHD diagnosis, and to consider any sex differences. Longitudinal population data from young people in the Millennium Cohort Study (UK) and the Child and Adolescent Twin Study in Sweden were used. Chapters 2 and 3 compared young people with earlier and later diagnosed ADHD, as well as undiagnosed and diagnosed ADHD. Chapter 4 compared young people with self and parent-rater agreement and disagreement on ADHD symptoms. Associations with various factors were tested, including mental health and neurodevelopmental conditions, social and relationship factors, and ADHD genetic risk. Findings indicated that general emotional and behavioural difficulties were associated with earlier ADHD diagnosis, but specific mental health symptoms, diagnosis, and prescriptions were associated with later diagnosed ADHD and rater disagreement. Mental health and neurodevelopmental conditions were associated with diagnosed ADHD vs undiagnosed ADHD. Better prosocial skills were associated with later recognition, undiagnosed ADHD and rater disagreement. Increased ADHD genetic risk was associated with diagnosed ADHD and rater agreement, but not diagnostic timing. Diagnosed males had higher emotional dysregulation than undiagnosed males, but this was not seen in females. Females were more likely to be diagnosed later, and to have rater disagreement. This thesis highlights that many young people could have undiagnosed ADHD. Clinicians, parents/carers and teachers should be aware that certain factors (e.g., better social skills or mental health problems) could be masking or overshadowing ADHD, and where ADHD symptoms occur with impact, further screening is warranted. Access to management (e.g. medication) and improved outcomes is most accessible with an ADHD diagnosis, highlighting this works importance.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Medicine
Date of First Compliant Deposit:	20 March 2026
Last Modified:	20 Mar 2026 16:43
URI:	https://orca.cardiff.ac.uk/id/eprint/185896

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