• Include in every autism diagnostic assessment:

1. detailed questions about parent's or carer's concerns and, if appropriate, the child's or young person's concerns
2. details of the child's or young person's experiences of home life, education and social care
3. a developmental history, focusing on developmental and behavioural features consistent with ICD-11 or DSM-5 criteria (consider using an autism-specific tool to gather this information)
4. assessment (through interaction with and observation of the child or young person) of social and communication skills and behaviours, focusing on features consistent with ICD-11 or DSM-5 criteria (consider using an autism-specific tool to gather this information)
5. a medical history, including prenatal, perinatal and family history, and past and current health conditions
6. a physical examination
7. consideration of the differential diagnosis 
8. systematic assessment for conditions that may coexist with autism 
9. development of a profile of the child's or young person's strengths, skills, impairments and needs that can be used to create a needs-based management plan, taking into account family and educational context.
10. communication of assessment findings to the parent or carer and, if appropriate, the child or young person. 

• Perform a general physical examination and look specifically for:
• skin stigmata of neurofibromatosis or tuberous sclerosis using a Wood's light
• signs of injury, for example self-harm or child maltreatment
• congenital anomalies and dysmorphic features including macrocephaly or microcephaly.

1. Consider the following differential diagnoses for autism and whether specific assessments are needed to help interpret the autism history and observations: Neurodevelopmental disorders ; specific language delay or disorder; a learning (intellectual) disability or global developmental delay ; developmental coordination disorder (DCD).

other differentials to be considered : Mental and behavioural disorders:

attention deficit hyperactivity disorder (ADHD)

mood disorder

anxiety disorder

attachment disorders

oppositional defiant disorder (ODD)

conduct disorder

obsessive compulsive disorder (OCD)

psychosis.

Conditions in which there is developmental regression:

Rett syndrome

epileptic encephalopathy.

Other conditions:

severe hearing impairment

severe visual impairment

maltreatment

selective mutism. 

• Consider which assessments are needed to construct a profile for each child or young person, for example:

1. intellectual ability and learning style
2. academic skills
3. speech, language and communication
4. fine and gross motor skills
5. adaptive behaviour (including self-help skills)
6. mental and emotional health (including self-esteem)
7. physical health and nutrition
8. sensory sensitivities
9. behaviour likely to affect day-to-day functioning and social participation
10. socialisation skills. 

• If there are discrepancies during the autism diagnostic assessment between reported signs or symptoms and the findings of the autism observation in the clinical setting, consider:

1. gathering additional information from other sources and/or
2. carrying out further autism‑specific observations in different settings, such as the school, nursery, other social setting or at home.
3. Use information from all sources, together with clinical judgment, to diagnose autism based on ICD-11 or DSM-5 criteria. 

Do not rely on any autism-specific diagnostic tool alone to diagnose autism. 

Be aware that in some children and young people there may be uncertainty about the diagnosis of autism, particularly in:

children younger than 24 months

children or young people with a developmental age of less than 18 months

children or young people for whom there is a lack of available information about their early life (for example, some looked-after or adopted children)

older teenagers

children or young people with a complex coexisting mental health disorder (for example ADHD, conduct disorder, a possible attachment disorder), sensory impairment (for example severe hearing or visual impairment), or a motor disorder such as cerebral palsy. 

• Be aware that some children and young people will have features of behaviour that are seen in the autism spectrum but do not reach the ICD‑11 or DSM-5 diagnostic criteria for definitive diagnosis. Based on their profile, consider referring to appropriate services.
• If the outcome of the autism diagnostic assessment clearly indicates that the child or young person does not have autism, consider referring them to appropriate services based on their profile.

Consider whether the child or young person may have any of the following as a coexisting condition, and if suspected carry out appropriate assessments and referrals:

Mental and behaviour problems and disorders:

ADHD

anxiety disorders and phobias

mood disorders

oppositional defiant behaviour

tics or Tourette syndrome

OCD

self-injurious behaviour.

Neurodevelopmental problems and disorders:

global delay or a learning (intellectual) disability

motor coordination problems or DCD

academic learning problems, for example in literacy or numeracy

speech and language disorder.

Medical or genetic problems and disorders:

epilepsy and epileptic encephalopathy

chromosome disorders

genetic abnormalities, including fragile X

tuberous sclerosis

muscular dystrophy

neurofibromatosis.

Functional problems and disorders:

feeding problems, including restricted diets

urinary incontinence or enuresis

constipation, altered bowel habit, faecal incontinence or encopresis

sleep disturbances

vision or hearing impairment. 

• Be aware that in children and young people with communication difficulties it may be difficult to recognise functional problems or mental health problems.

Recommendations | Autism spectrum disorder in under 19s: recognition, referral and diagnosis | Guidance | NICE