A lot of spectrum disorder diagnoses are misdiagnoses for other (medical) things. The diagnoses are often used as a catchall when no one can figure it out.

For example, chronic heavy metal poisoning mimicks ADHD or ADD diagnoses, blood tests alone are useless because its bound to tissue very quickly after exposure (and bioaccumulates unless a chelator is taken). Toxicity for a number of other substances also presents similarly to these diagnoses (flouride, pfas, microplastics).