Is the diagnose of autism always accurate in a child, or do many other disorders cause autism behaviors?

Autistic Disorder is. A categorical diagnosis of a set of observable behaviors with many different etiologies, or causes. The underlying deficits that cause difficullty in joint attention, gaze monitoring, pretend play, face recognition, processing social/emotional information, social communication, social reciprocity & stereotyped, repetitive movements or narrow, restricted areas of interest are unique to ad.
Requires expertise. Autism can present in many forms, and other disorders can have elements that can be mistaken for it. Age also is important: 2 or 3 yo children present much differently than older ones or adolescents. No behaviors associated with autism occur only with autism. Experts look at the total picture, both how development is going and what impairments are present. So, far no biological tests.