[flagged]
ADHD is diagnosed by looking at just symptoms if I remember correctly. I’m not sure about autism.
I knew someone who became sick. At first it was thought to be Lyme, it took a long while to determine and a bunch of other theories to determine it was lupus. Tests were not fully accurate and these illnesses manifest a little differently in different people.
In the case of my children, there were certain tests that were absolutely definitive, and not reliant on interpretation - namely, idioms and metaphors. It's raining cats and dogs, for example.
https://peterattiamd.com/trennasutcliffe/
At the more severe end of the spectrum, diagnoses tend to be more objective and consistent.