A positive screen is not a diagnosis. ADHD is one of the most over- and under-diagnosed conditions in child health at the same time because so many other things can look like it. This case-based guide shows how to separate ADHD from autism, anxiety, OCD, specific learning disorder and ODD.
Key clinical takeaways
- DSM-5 loosened the criteria: onset moved from under 7 to under 12, fewer symptoms are needed in adults, and ADHD plus autism can now be diagnosed together.
- DSM criteria describe impairment, not cause. Difficulty sustaining attention can reflect distraction, anxiety, or hyper-focus.
- ADHD can be understood as an under-aroused brain seeking stimulation; autism as an over-aroused brain seeking calm.
- Rating scales such as SNAP, Conners, ADHD-RS and SDQ support screening and monitoring, but they do not diagnose ADHD.
- Hold a lower threshold for ADHD in girls, looked-after children, preterm birth, epilepsy, ODD/conduct presentations and youth-justice contact.
Why ADHD diagnoses have surged: DSM-4 vs DSM-5
To understand today's ADHD population, clinicians need to understand the diagnostic history. Under DSM-4, criteria were stricter: age of onset had to be before 7, the symptom threshold was the same for adults and children, and ADHD could not be diagnosed alongside autism.
That last rule distorted clinical understanding. If a child had both autism and ADHD but only ADHD was severe, they were often labelled ADHD and autistic traits could be absorbed into that label. DSM-5 changed this: onset moved to before 12, fewer symptoms are required in adults, and ADHD and autism can be diagnosed together. This captured milder cases, including many girls who were previously missed, while also increasing the need for careful differential diagnosis.
| DSM-4 | DSM-5 | |
|---|---|---|
| Age of onset | Before 7 years | Before 12 years |
| Symptom threshold | Same for adults and children | Lower for adults |
| With autism? | Cannot co-diagnose | Can co-diagnose |
| Effect | Milder cases and girls missed | Milder cases caught; over-diagnosis risk if the differential is weak |
The core frame: what an ADHD brain is doing
A useful clinical model is that the ADHD brain is under-aroused and seeks stimulation and novelty. This explains why fidgeting may help focus, why low-stimulation tasks feel harder, why attention is pulled toward reward, and why delay aversion can drive impulsivity.
Contrast this with the autistic brain, which is often over-aroused and seeks calm. Holding those two opposite drives in mind helps the differential become clearer.
Case-based differential diagnosis
Six fictional cases show conditions that can mimic ADHD and how to think through the difference.
"A handful," very hyperactive, no nursery concerns. Avoid diagnosing ADHD before the fifth year. High distractibility and activity are normal under 4, and developmental age matters if there is delay.
Bright, well-liked, performance fell in the last year. Intelligence can mask ADHD until organisational demands increase. Symptoms may have been present before 12, while impairment appears later.
Clever but struggles with reading and writing, fine at maths and chess. Think specific learning disorder. ADHD performance is often inconsistent and motivation-linked; SLD is more domain-specific.
Extreme hyperactivity with a set pattern, flapping and sensory distress. Autistic hyperactivity is often repetitive and self-calming; ADHD hyperactivity is more random and exploratory.
Inattention without hyperactivity, perfectionism and sensory hypersensitivity. Inattention in girls is not automatically ADHD. Consider autism and OCD, especially where attention gets stuck rather than jumps.
Mind never at rest, insomnia, fidgety and anxious. In ADHD, inattention usually pre-dates anxiety and is trait-like; in anxiety, inattention worsens with anxious states and thoughts are often negative worries.
ADHD vs autism: hyperactivity
| ADHD hyperactivity | Autistic hyperactivity | |
|---|---|---|
| Underlying drive | Under-aroused and seeking stimulation | Over-aroused and seeking calm |
| Quality | Random, exploratory, varied | Repetitive, set patterns |
| When | Trait-like and mostly present | Often state-linked when overwhelmed |
| In clinic | Explores different things in the room | Repeats the same movement or behaviour |
ADHD vs autism: attention
| ADHD attention | Autistic attention | |
|---|---|---|
| Pattern | Jumps between tasks; careless mistakes | Gets stuck on one thing |
| Detail | Skips detail | Excessive detail; fixed interests |
| Distraction | Pulled toward pleasing or rewarding stimuli | Distracted by stimuli it dislikes |
ADHD vs anxiety
| ADHD | Anxiety | |
|---|---|---|
| Onset | Inattention pre-dates anxiety | Inattention follows anxiety |
| Inattention | Trait-like and consistently present | State-like and worse when anxious |
| Thoughts | Random, positive, negative or mixed | Negative worries and rumination |
| Fidgeting | To become alert | To calm down |
How ADHD is actually diagnosed
The diagnosis is clinical. A thorough developmental history establishes onset before 12, a pervasive and continuous course across at least two settings, and clear impairment. Rating scales support screening, baselining and monitoring, but do not clinch the diagnosis.
Structured interviews such as DAWBA and K-SADS add rigour and can help clinicians learn history-taking. QbTest can be a useful adjunct where collateral or history is limited. Always seek multiple informants, and when school and home disagree, ask why.
Red flags and when to refer
Prioritise sudden onset or regression, severe impairment or risk, safeguarding concerns, self-harm, aggression, or signs of secondary causes such as seizure or brain insult. Cardiac history does not change the diagnosis, but it changes treatment options. Be cautious if symptoms appear in only one setting.
For clinicians moving into ADHD practice
Watching a webinar builds awareness, but competence needs structured learning, supervision and assessed practice. MEDLRN training focuses on developing the competencies needed to work safely in ADHD.
Explore ADHD courseFrequently asked questions
How do you tell ADHD apart from autism in a child?
Look at the nature of hyperactivity and attention. ADHD usually shows random, exploratory hyperactivity and attention that jumps. Autism more often shows repetitive, self-calming hyperactivity and attention that gets stuck.
How do you differentiate ADHD from anxiety?
Take a developmental history. In ADHD, attention difficulties usually pre-date anxiety and are trait-like. In anxiety, inattention worsens with anxious states and thoughts are usually negative worries.
Why have ADHD diagnoses increased?
DSM-5 changed the criteria: onset before 12 rather than 7, fewer symptoms required in adults, and ADHD plus autism can now be diagnosed together.
Do rating scales diagnose ADHD?
No. They support screening and monitoring. Diagnosis rests on clinical interview, developmental history, impairment and differential diagnosis.
Sources and further reading
- NICE NG87: Attention deficit hyperactivity disorder: diagnosis and management
- Royal Pharmaceutical Society: A Competency Framework for all Prescribers
- American Psychiatric Association, DSM-5 diagnostic criteria for ADHD.