This guideline makes recommendations for the diagnosis and management of attention deficit hyperactivity disorder (ADHD) in children, young people and adults. The guideline does not cover the management of ADHD in children younger than 3 years. The term 'children' refers to those aged 11 years and younger; 'young people' refers to those between 12 and 18 years. However, these categories are flexible and clinicians should use their judgement about a child or young person's developmental, as opposed to their chronological, age.

ADHD is a heterogeneous behavioural syndrome characterised by the core symptoms of hyperactivity, impulsivity and inattention. While these symptoms tend to cluster together, some people are predominantly hyperactive and impulsive, while others are principally inattentive. Two main diagnostic criteria are in current use – the International Classification of Mental and Behavioural Disorders 10th revision (ICD-10) and the Diagnostic and Statistical Manual of Mental Disorders 4th edition (DSM-IV). ICD-10 uses a narrower diagnostic category, which includes people with more severe symptoms and impairment. DSM-IV has a broader, more inclusive definition, which includes a number of different ADHD subtypes. Although ICD-10 excludes any comorbidity, for the purposes of this guideline coexisting conditions are accepted as a common aspect of the diagnosis and treatment of ADHD.

Symptoms of ADHD are distributed throughout the population and vary in severity; only those with significant impairment meet criteria for a diagnosis of ADHD. Symptoms of ADHD can overlap with symptoms of other related disorders, and ADHD cannot be considered a categorical diagnosis. Therefore care in differential diagnosis is needed. Common coexisting conditions in children with ADHD are disorders of mood, conduct, learning, motor control and communication, and anxiety disorders; in adults they include personality disorders, bipolar disorder, obsessive-compulsive disorder and substance misuse. As a result, ADHD cannot be considered a categorical diagnosis.

Not every person with ADHD has all of the symptoms of hyperactivity, impulsivity and inattention. However, for a person to be diagnosed with ADHD, their symptoms should be associated with at least a moderate degree of psychological, social and/or educational or occupational impairment. For the purposes of this guideline, where the word 'impairment' is used in the recommendations, it means 'psychological, social and/or educational or occupational impairment'.

Moderate ADHD in children and young people is taken to be present when the symptoms of hyperactivity/impulsivity and/or inattention, or all three, occur together, and are associated with at least moderate impairment, which should be present in multiple settings (for example, home and school or a healthcare setting) and in multiple domains (domains refers to a type of social or personal functioning in which people ordinarily achieve competence, such as, achievement in schoolwork or homework; dealing with physical risks and avoiding common hazards; and forming positive relationships with family and peers), where the level appropriate to the child's chronological and mental age has not been reached. Determining the severity of the disorder should be a matter for clinical judgement, taking into account the severity of impairment, pervasiveness, individual factors and familial and social context.

The level of impairment could also be estimated by using a predetermined level on a global adjustment scale (for example, a score of less than 60 on the children's global assessment scale [C-GAS]).

In later adolescence and adult life, the range of possible impairment extends to educational and occupational underachievement, dangerous driving, difficulties in carrying out daily activities such as shopping and organising household tasks, in making and keeping friends, in intimate relationships (for example, excessive disagreement) and with childcare.

Severe ADHD corresponds approximately to the ICD-10 diagnosis of hyperkinetic disorder. This is defined as when hyperactivity, impulsivity and inattention are all present in multiple settings, and when impairment is severe (that is, it affects multiple domains in multiple settings). Again, determining severity is a matter of clinical judgement.

For the sake of clarity, the Guideline Development Group has examined the validity of the diagnosis of ADHD, and advice is given about diagnosis in the recommendations.

Based on the narrower criteria of ICD-10, hyperkinetic disorder is estimated to occur in about 1–2% of children and young people in the UK. Using the broader criteria of DSM-IV, ADHD is thought to affect about 3–9% of school-age children and young people in the UK, and about 2% of adults worldwide.

In general, ADHD is a persisting disorder. Of the young people with a sustained diagnosis, most will go on to have significant difficulties in adulthood, which may include continuing ADHD, personality disorders, emotional and social difficulties, substance misuse, unemployment and involvement in crime.