This guideline was updated in 2017 to include attention deficit hyperactivity disorder (ADHD) in the list of factors that should be taken into account when considering a referral for possible autism spectrum disorder and when considering whether to carry out an autism spectrum disorder diagnostic assessment. This is because there is evidence showing an increased prevalence of autism spectrum disorders in children and young people with ADHD. In addition, it is important that clinicians do not exclude or delay referral for possible autism spectrum disorder because of an earlier diagnosis of ADHD.

The term autism describes qualitative differences and impairments in reciprocal social interaction and social communication, combined with restricted interests and rigid and repetitive behaviours. Autism spectrum disorders are diagnosed in children, young people and adults if these behaviours meet the criteria defined in the International Statistical Classification of Diseases and Related Health Problems (ICD-11) and the Diagnostic and Statistical Manual of Mental Disorders Fifth Edition (DSM-5) and have a significant impact on function. The overarching category term used in ICD-11 is pervasive developmental disorder (PDD), a term now used synonymously with autism spectrum disorder (excluding Rett's syndrome); it is a behaviourally defined group of disorders, which is heterogeneous in both cause and manifestation.

DSM-5 refers to autism spectrum disorder as a single condition with different levels of symptom severity in 2 core domains: 1) deficits in social communication and social interaction and 2) restricted repetitive behaviours (RRBs), interests, and activities and sensory anomalies. Autism spectrum disorder encompasses the 4 separate disorders from the previous DSM-IV: autistic disorder (autism), Asperger's disorder, childhood disintegrative disorder and pervasive developmental disorder not otherwise specified. Because both components are required for diagnosis of autism spectrum disorder, social (pragmatic) communication disorder is diagnosed if no RRBs are present. Note that childhood disintegrative disorder is excluded from ASD in DSM-5.

The guideline development group recognised that individuals and groups prefer a variety of terms, including autism spectrum disorder, autistic spectrum condition, autistic spectrum difference and neuro-diversity. For clarity and consistency, in this guideline the term 'autism' is used throughout, in keeping with the use of 'autism' in the government's 2010 strategy for adults with autism. However in this guideline 'autism' refers to 'autism spectrum disorders'.

Autism is a lifelong disorder that has a great impact on the child or young person and their family or carers. When autism is diagnosed, families and carers and the child or young person themselves can experience a variety of emotions, shock and concern about the implications for the future. They may also have a profound sense of relief that others agree with their observations and concerns. Diagnosis and the assessment of needs can offer an understanding of why a child or young person is different from their peers and can open doors to support and services in education, health services and social care, and a route into voluntary organisations and contact with other children and families with similar experiences. All of these can improve the lives of the child or young person and their family.

The core autism behaviours are typically present in early childhood, but features are not always apparent until the circumstances of the child or young person change, for example when the child goes to nursery or primary school or moves to secondary school. Autism is strongly associated with a number of coexisting conditions. Recent studies have shown that approximately 70% of autistic people also meet diagnostic criteria for at least one other (often unrecognised) psychiatric disorder that is further impairing their psychosocial functioning. A learning (intellectual) disability (intelligence quotient [IQ] below 70) occurs in approximately 50% of young autistic people.

Autism was once thought to be an uncommon developmental disorder, but recent studies have reported increased prevalence and the condition is now thought to occur in at least 1% of children. This rising prevalence has increased demand for diagnostic services for children and young people of all ages in the health service.

Health services have a key role in recognising and diagnosing autism. Levels of understanding of autism among healthcare and other relevant professionals and availability of services differ greatly from one area to another. In addition, children and young people with certain coexisting conditions such as a learning (intellectual) disability are less likely to be diagnosed with autism, leading to inequalities in healthcare and service provision.

Coordination between health agencies and other key services such as education, social care and the voluntary sector is important. Multi-agency staff should also work in partnership with the autistic child or young person and their family or carers.

This guideline aims to improve recognition, referral and diagnosis, and the experience of children, young people and those who care for them. NICE has also produced a guideline on autism spectrum disorder in under 19s: support and management.