The distinctions between these diagnoses hinge on subtle differences in language development, severity, and age of onset (age at which symptoms first appeared), as well as other features that a health care provider can evaluate. For instance, the diagnosis of autistic disorder requires symptoms in all three of the areas described above, as well as a history of a delay in developing spoken language and onset of symptoms prior to 36 months of age. By contrast, Asperger disorder is an appropriate diagnosis for individuals who show many of the same features of Autistic disorder in all three areas, but who specifically do not have any delay in developing spoken language. Pervasive developmental disorder not otherwise specified (PDD/NOS) is a diagnosis assigned to children who have symptoms in all three areas, but who do not meet the specific diagnostic criteria for any of the other pervasive developmental disorders.

Parents often tell us their child has received several different PDD diagnoses, which may be due to changes in symptoms over time or variability in the methods of assessment. These diagnoses may include the five listed above, or others (not officially recognized medical diagnoses) such as "autism spectrum condition," "high functioning autism," or atypical autism." The only way to diagnose someone with a PDD is through clinical means (such as observation and interviews) — there is no "medical test" that can give a clear diagnosis.

Asperger Disorder
Asperger disorder, sometimes referred to as Asperger syndrome, is a neurobiological disorder. In contrast to autistic disorder, individuals with Asperger disorder do not have a delay in spoken language development. However, they can have serious deficits in social and communication skills. They often have obsessive, repetitive routines and preoccupations with a particular subject matter.

Pervasive Developmental Disorder Not Otherwise Specified (PDD/NOS)
PDD/NOS is a diagnosis often considered for children who show some signs of autistic disorder, but who do not meet the specific diagnostic criteria for the other PDDs.

Rett Disorder
Rett disorder is a complex neurological disorder that affects mainly girls, but there are reports of males who this disorder. Rett disorder is genetic in origin, and is among the most common genetic cause of profound intellectual and physical disability in girls, occurring more commonly than 1 in 10,000 female births. Individuals with Rett disorder develop normally until 6 to 18 months of age followed by a developmental regression. This regression is followed by a deceleration of head growth, loss of purposeful hand movements and followed by the appearance of midline, stereotypic hand movements. A gene associated with Rett disorder was identified in 1999.

Childhood Disintegrative Disorder
Children with childhood disintegrative disorder develop normally for a relatively prolonged period (usually 2 to 4 years) before developing a condition that resembles autistic disorder. Typically language, interest in the social environment, and often toileting and self-care abilities are lost, and there may be a general loss of interest in the environment.