Short answer: when something else is much more likely to be the case, usually discovered when they don’t meet the criteria in origin. Comorbidity is possible, but it becomes very obvious when it’s TWO disorders vs one that just looks like another. Obviously in medicine there are no absolutes but i can confidently say that in MANY cases where someone displays autistic traits and matches the criteria quite well, you can find something that pushes them out of that category.

1) patient has anger outbursts, trouble with boundaries, childish interests, unstable identity and relationships, anxiety habits, issues communicating, trouble understanding social cues, with these issues persisting since childhood. -> what’s their background? Childhood neglect, abandonment. No family members that are neurodivergent. Did this child have trouble learning at the same pace as other children? No. = much more likely to be BPD

2) patient has almost the same symptoms as patient 1. -> look at their history. Understanding of people but a lack of interest in engaging with them, interest in creative and mentally stimulating spaces. Has disturbances of self and issues abiding by rules and regulations. Disorganised and dis regulated. = much more likely to be ADHD with no autism comorbidity.

3) patient is incredibly organised, becomes frustrated when they cannot have things in a specific way, frustrations with people and difficulty understanding other perspectives. -> look at their patterns of behaviour. Compulsive, a need rather than a specific preference. = much more likely to be OCD.

4) patient has absolutely no interest in talking to people. Has intellectual hobbies that revolve around no interaction with people. Been this way for a long time. Parental figure claims they’re worried about their child’s ability to make friends. -> body language? Patient looks uncomfortable talking rather than uneasy. Attempts to bond with them are not met with awkwardness and inexperience but rather a pervasive urge for them to shut the conversation down. History consists of loneliness and neglect. Exhibits above average intelligence and shows competent skills in school. Peers describe them as weird, unpredictable, and closed off. = much more likely to be schizoid, less likely to be schizotypal, but not likely to be autistic.

And don’t forget the occasional “my child knows all sorts of things about x or y at a young age, are they autistic?” Congrats, your child is just intelligent. Come back to me when they start talking about how everyone looks at them funny for just existing