Currently, there is no single medical test that will definitively diagnose autism. Instead, the diagnosis is made on the basis of observable characteristics of the individual.
Because most children start showing symptoms of autism at about 18-24 months, British researchers have developed a screening tool called the Checklist for Autism in Toddlers . The CHAT (which takes five minutes to administer) has been shown to be highly effective in predicting which children will develop autism, PDD, Asperger's or other developmental syndromes.
Before embarking on any sort of diagnosis, however, it is important to rule out possible physical causes for an individual's behavior. Please consult your physician or pediatrician if you have concerns about a family member or friend.
Official Diagnostic Criteria
The Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) was revised in 1994. Slight changes were made to the diagnostic criteria for autism, and the category of pervasive developmental disorders (PDD) which includes autism was reclassified from a "long-term stable disorder with poor prognosis" to the more transient "temporary episodic clinical disorder." This positive move reflects the facts that there is a possibility of improvement with intervention, and that symptoms of these disorders vary in intensity.
The specific diagnoses used for autism and related disorders are:
Autistic disorder (classic autism): Onset occurs before age three. The child shows impairment in the three areas of observable symptoms: difficulty in communication, social interaction and repetitive, stereotyped patterns of behavior.
Childhood Disintegrative Disorder: The child develops normally in all areas for the first two years, then shows a significant loss of previously acquired skills.
Rett's Disorder (also known as Rett Syndrome): Found almost exclusively in females, the child achieves normal development for the first 5 months, then loses previously acquired communication skills and the purposeful use of the hands. These losses are soon followed by other areas of deterioration, including apraxia (loss of ability to control complex muscle movements), gait disturbances and sometimes seizures.
Asperger's Disorder (also known as Asperger's Syndrome): Children with this disorder demonstrate average to above-average in intelligence and no significant delay in language, but show impairment in social interactions and have a restricted range of interests and activities.
Pervasive Developmental Disorder, Not Otherwise Specified (atypical autism): In the case of PDD-NOS, there is severe impairment in specified behaviors, but the child does not meet the criteria for a specific diagnosis.