Differential Diagnosis in Adult ADHD: Common Pitfalls and Clinical Reasoning
Assessment of ADHD in adults requires careful consideration of alternative explanations for attentional, behavioural, and executive functioning difficulties. Many psychiatric, medical, and psychosocial conditions can present with symptoms that overlap with ADHD, increasing the risk of misdiagnosis if differential diagnosis is not systematically addressed. NICE guidance emphasises that adult ADHD should only be diagnosed when symptoms are not better explained by another condition. This article outlines key differential diagnoses to consider during adult ADHD assessment and highlights common clinical pitfalls.
Why Differential Diagnosis Is Central to Adult ADHD Assessment
Adult ADHD assessment is not a screening exercise and should not rely solely on symptom checklists. NICE guideline NG87 makes clear that ADHD should not be diagnosed if presenting symptoms are better accounted for by another mental health condition, medical disorder, substance use, or situational stressor. Failure to adequately consider differential diagnoses can result in inappropriate treatment, missed underlying pathology, and medico-legal vulnerability
Anxiety Disorders and Mood Disorders
Anxiety disorders and depressive disorders frequently present with concentration difficulties, restlessness, reduced motivation, and impaired executive functioning. In contrast to ADHD, attentional difficulties related to anxiety or mood disorders are often episodic and context-dependent, fluctuating with symptom severity. NICE-aligned assessment requires careful exploration of symptom chronology, persistence, and whether attentional difficulties pre-date the onset of affective symptoms.
Bipolar Disorder and Episodic Mood Elevation
Bipolar affective disorder may overlap with ADHD through symptoms such as impulsivity, distractibility, and increased activity. However, bipolar disorder is characterised by episodic mood elevation and behavioural change, whereas ADHD symptoms are typically lifelong and non-episodic. NICE guidance emphasises the importance of distinguishing chronic neurodevelopmental patterns from episodic mood disorders when assessing adults for ADHD.
Trauma-Related and Stress-Related Conditions
Post-traumatic stress disorder and other trauma-related conditions can impair attention, memory, and emotional regulation. In these cases, attentional difficulties are often threat-driven and associated with hyperarousal, avoidance, or intrusive symptoms. NICE-aligned assessment requires careful consideration of trauma history and symptom triggers to avoid misattributing trauma-related attentional difficulties to ADHD.
Autism Spectrum Condition and Neurodevelopmental Overlap
Autism spectrum condition commonly co-occurs with ADHD, but the two conditions have distinct core features. Difficulties with attention and executive functioning in autism are often linked to cognitive rigidity, sensory processing differences, and social communication challenges. Differential diagnosis should consider whether attentional difficulties arise from primary neurodevelopmental differences rather than ADHD-specific symptomatology.
Sleep Disorders, Substance Use, and Medical Causes
Sleep disorders, including obstructive sleep apnoea and chronic insomnia, can significantly impair attention, concentration, and daytime functioning. Substance use and medication effects may also mimic ADHD symptoms. NICE guidance requires that physical health factors, sleep quality, and substance use are actively explored and addressed before confirming an ADHD diagnosis.
Clinical Judgement and Medico-Legal Robustness
Differential diagnosis in adult ADHD relies on careful synthesis of clinical information rather than checklist-based exclusion. Clear documentation of alternative diagnoses considered, evidence supporting or refuting them, and the rationale for diagnostic conclusions is essential. NICE-aligned documentation strengthens continuity of care and ensures diagnostic decisions are clinically defensible.
An overview of our CPD-certified approach to adult ADHD assessment and diagnosis is available on the homepage.
This framework has been developed by ADHD Clinical Academy and is provided for educational purposes, reflecting a NICE-aligned approach to adult ADHD assessment. Further structured teaching, applied clinical examples, and comprehensive guidance on assessment, differential diagnosis, and clinical documentation are covered within our CPD- certified adult ADHD assessment training.
This content is intended for healthcare professionals and does not replace formal clinical training, supervision, or adherence to local governance requirements.