ASD (autism spectrum disorder) is a heterogeneous condition with no single pathognomonic feature or specific diagnostic test. Diagnosis can be challenging as affected individuals display variation in the degree of behavioural severity, language and intellectual abilities. Moreover, their behavioural profiles are likely to change with age and cooccurring problems and co-morbidities are common. DSM-5 recognises that symptoms in the early developmental period may not manifest until capabilities are exceeded by social demands. Similarly, there is recognition that for some adolescents, repetitive behavioural manifestations are reduced through developmental progress or intervention so criterion can be met based on history. For a diagnosis of ASD under ICD-10, abnormal or impaired development should be present by the age of 3 years.
Many parents express concerns as early as 15-18 months of age, but despite increased awareness and guidance, average age at diagnosis remains at 4-5 years. This is possibly due to a combination of factors that include variability of assessment pathways, demand on services, lack of recognition of subtle difficulties at a young age, presence of additional diagnoses and inclusion of school age individuals who may only present at an older age when their difficulties may become more overt as they are unable to manage increasingly challenging academic and/or social expectations. Studies have shown that diagnosis of ASD at 2 years of age is possible and stable over time, although it is less reliable for the broader autism spectrum.