Diagnosing an autistic spectrum disorder (ASD) in an individual is often a challenging process. Most of the children look physically normal; while even minor congenital abnormalities often do not adversely affect their appearance.

Currently, there are no medical tests which can be used to diagnose autism, nor tell the difference between sub-groups within the autistic spectrum disorders (ASD). In other words, blood tests, X-rays, brain scans, electroencephalograms (records of electrical waves from the brain) and other physical examinations cannot confirm if a child has ASD.

Even the recently developed "Theory of Mind" psychological tests, although helpful, cannot be used to confirm or refute the presence of ASD. It has been noted by Lorna Wing in "Autistic Spectrum Disorders – an aid to diagnosis" that ‘even the theory of mind tests are often passed by more able, older children and adults, despite their lack of empathy with others in real life.’

The diagnosis of ASD is based largely on behaviour. Internationally, experts use certain behavioural criteria for the diagnosis of autism. The most detailed and most recent scheme is the one described in Diagnostic and Statistical Manual (DSM-IV) of the American Psychiatric Association. A similar diagnostic scheme is available in the International Classification of Diseases (ICD-10) issued by the World Health Organisation.

The essential criteria, accompanied by specific examples, are classified under the following headings:

Qualitative impairment in reciprocal social interaction

Qualitative impairment in verbal and non-verbal communication and in imaginative activity

Markedly restricted repertoire of activities and interests.

It is important for the individual and/ or his family to compile a systematic and detailed personal history of the individual and his behaviour, ideally starting from birth/ infancy until the present. Usually, a group of different doctors and health professionals, such as psychiatrists, clinical psychologists, and paediatricians would be involved in the diagnosis. They typically ask the individual a series of relevant questions, make observations on the individual’s behaviour, as well as administer a range of psychological tests. This takes some time to complete, "at least two to three hours for the interview alone", writes Lorna Wing, in "The Autistic Spectrum – A Guide for Parents and Professionals". Importantly, the procedure should be systematic and unhurried to prevent misdiagnosis.

Autism is fairly rare, and there are relatively few experts who have the experience of a large number of cases. Hence, for diagnosticians, experience with ASD is a major plus factor, as it allows them to quickly sense that characteristic "autism aloneness". Interpreting the significance of deviant, absent or delayed behaviour also requires a sound background of clinical knowledge.

It should be noted ASD can occur alongside any other physical or psychological challenge. If a child has Down’s syndrome, cerebral palsy, a hearing or visual impairment, or other chromosomal abnormalities, it does not mean that s/he cannot have ASD as well. Similarly, ASD can and often does co-exist with dyslexia, language disorders, or any other type of specific or general learning disability. Hence, asking ‘Is this autism or a language disorder, deafness, etc?’, is quite useless. A child may have a language disorder, deafness, etc. and could have ASD as well.

Conversely, when an autistic spectrum disorder is being diagnosed, the possibility of accompanying physical or psychological challenges should be considered as well.

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