ADHD and Differential Diagnosis: Why a Comprehensive Assessment Matters

Many of the difficulties associated with Attention Deficit Hyperactivity Disorder (ADHD) are not unique to ADHD. Difficulties with attention, concentration, memory, emotional regulation, organisation and restlessness can occur in a wide range of mental health conditions, physical health conditions and neurodevelopmental differences.

This is one of the reasons why diagnosing ADHD requires a comprehensive assessment rather than simply completing a questionnaire or recognising a few familiar traits. An experienced clinician will carefully consider whether your presentation is best explained by ADHD, another condition, or a combination of factors.

What is a differential diagnosis?

A differential diagnosis is the process of considering other conditions that could explain a person's symptoms before reaching a final diagnosis.

Many conditions overlap with ADHD and share similar characteristics. In some cases, people may receive an ADHD diagnosis after years of being treated for anxiety or depression. In other cases, difficulties initially thought to be ADHD are better explained by another condition.

The goal is not simply to identify symptoms but to understand why those symptoms are occurring.

Why can ADHD look like other conditions?

ADHD affects attention, executive functioning and self regulation. These same areas can also be affected by many other conditions.

For example, someone who struggles to concentrate may have ADHD, but they may also be exhausted, anxious, traumatised, depressed or experiencing the effects of chronic pain.

Looking only at the symptom without understanding its cause can lead to an incorrect diagnosis.

Anxiety

Anxiety can significantly affect concentration and memory.

When someone is constantly worrying or anticipating potential problems, much of their mental energy is focused on managing those thoughts rather than the task in front of them.

People experiencing anxiety may:

• Appear distracted.

• Forget information.

• Struggle to finish tasks.

• Avoid demanding activities.

• Find it difficult to organise themselves.

These difficulties can resemble ADHD, but they are often driven by excessive worry rather than lifelong differences in attention regulation.

It is also important to recognise that anxiety commonly occurs alongside ADHD. Many people have both conditions.

Trauma

Trauma can have a profound effect on attention, emotional regulation and memory.

People who have experienced trauma may appear hypervigilant, easily distracted or emotionally reactive. They may find it difficult to remain focused because their brain is prioritising safety over concentration.

Some people also experience:

• Dissociation.

• Poor working memory.

• Sleep disturbance.

• Emotional overwhelm.

• Restlessness.

These difficulties can closely resemble ADHD, particularly if the history of trauma has not been fully explored.

However, trauma and ADHD can also occur together, making assessment more complex.

Depression

Depression affects much more than mood.

People experiencing depression often describe:

• Poor concentration.

• Reduced motivation.

• Forgetfulness.

• Slowed thinking.

• Difficulty making decisions.

These symptoms can sometimes be mistaken for inattentive ADHD, particularly if the assessment focuses only on current functioning.

A key difference is that ADHD begins in childhood, whereas depression usually develops later and represents a change from previous functioning.

Autism

Autism and ADHD frequently occur together, with many people meeting criteria for both conditions.

However, some autistic characteristics may resemble ADHD, including:

• Executive functioning difficulties.

• Forgetfulness.

• Difficulties organising tasks.

• Emotional regulation differences.

• Sensory overwhelm affecting concentration.

Understanding whether these difficulties are better explained by autism, ADHD or both requires careful assessment of developmental history and the individual's overall presentation.

Sleep disorders

Poor sleep is one of the most common causes of concentration difficulties.

Sleep deprivation can affect:

• Attention.

• Working memory.

• Emotional regulation.

• Processing speed.

• Impulse control.

People with untreated sleep disorders may appear forgetful, distracted and unable to sustain attention throughout the day.

Before diagnosing ADHD, clinicians should consider whether sleep difficulties may be contributing to the presentation.

Physical health conditions

A number of physical health conditions can affect concentration and thinking.

Examples include:

• Thyroid disorders.

• Iron deficiency.

• Vitamin deficiencies.

• Long COVID.

• Chronic fatigue syndrome (ME/CFS).

• Chronic pain conditions.

• Neurological conditions.

• Medication side effects.

These conditions may produce cognitive difficulties that resemble ADHD but have a different underlying cause.

Substance use

Alcohol, recreational drugs and some prescribed medications can affect attention, memory and executive functioning.

Understanding whether symptoms have always been present or developed following substance use is an important part of the assessment process.

Stress and burnout

Periods of prolonged stress or burnout can affect almost every aspect of thinking.

People experiencing burnout often describe:

• Brain fog.

• Poor concentration.

• Difficulty organising tasks.

• Reduced motivation.

• Increased forgetfulness.

Although these symptoms may resemble ADHD, they often improve when stress levels reduce and recovery takes place.

Why developmental history matters

One of the most important aspects of an ADHD assessment is understanding how someone functioned during childhood.

ADHD is a neurodevelopmental condition, which means symptoms begin during childhood, even if they are not recognised until adulthood.

A clinician will usually explore:

• Childhood behaviour.

• School experiences.

• Family history.

• Lifelong patterns of attention and organisation.

• Whether difficulties have been present across different settings.

This helps distinguish ADHD from conditions that develop later in life.

Can someone have more than one condition?

Yes.

It is common for people with ADHD to also experience:

• Anxiety.

• Depression.

• Autism.

• Trauma related difficulties.

• Obsessive compulsive disorder (OCD).

• Learning differences.

• Sleep disorders.

Having another diagnosis does not rule out ADHD, just as having ADHD does not prevent someone from developing other mental or physical health conditions.

A comprehensive assessment considers the whole person rather than looking for a single explanation.

Why a comprehensive assessment is essential

There is no blood test or brain scan that can diagnose ADHD.

Diagnosis is based on a detailed clinical assessment that considers developmental history, current difficulties, information from childhood where available, the impact on everyday life and whether another condition better explains the presentation.

A thorough assessment should never focus solely on symptoms. It should seek to understand why those symptoms occur and whether ADHD is the most appropriate explanation.

Final thoughts

Many conditions share features with ADHD, and it is not always straightforward to distinguish between them. Difficulties with attention, memory, emotional regulation and organisation can arise from anxiety, trauma, depression, sleep problems, physical health conditions and many other factors.

This is why a comprehensive ADHD assessment is so important. Rather than looking at individual symptoms in isolation, experienced clinicians consider a person's developmental history, current presentation, overall functioning and any alternative explanations before reaching a diagnosis. This careful approach helps ensure that people receive the most accurate diagnosis and the most appropriate support for their individual needs.