Diagnosing autism spectrum disorder (ASD) can be difficult because there is no medical test, such as a blood test, to diagnose the disorder. Doctors look at the child's developmental history and behavior to make a diagnosis.

ASD can sometimes be detected at 18 months of age or younger. By age 2, a diagnosis by an experienced professional can be considered reliable. However, many children do not receive a final diagnosis until they are much older.

Diagnosing children with ASD as early as possible is important to make sure children receive the services and support they need to reach their full potential. There are several steps in this process.

A brief test using a screening tool does not provide a diagnosis, but it can indicate whether a child is on the right development track or if a specialist should take a closer look. If the screening tool identifies an area of concern, a formal developmental evaluation may be needed.

This formal evaluation is a more in-depth look at a child's development and is usually done by a trained specialist such as a developmental pediatrician, child psychologist, speech-language pathologist, or occupational therapist. The results of a formal developmental evaluation can also show whether your child needs early intervention services. In some cases, the specialist might recommend genetic counseling and testing for your child.

The specialist may observe the child and give the child a structured test, ask the parents or caregivers questions, or ask the parents or caregivers to fill out questionnaires. The results of this formal evaluation highlight your child's strengths and challenges and can inform whether they meet the criteria for a developmental diagnosis.