Is it possible that a 33yr. old who has been diagnosed with adhd actually be autistic instead?

Yes it is possible: A solid neuropysychological evaluation would be able to tease out adhd from autism. So yes, it is certainly possible that someone diagnosed with adhd actually has autism instead ( or has both ). If an individual is exhibiting impaired social interactions, repetitive behaviors and inflexible adherence to rules, it would be good to obtain additional clinical assistance to clarify a diagnosis.

Each is a neuro-: developmental disorder with a unique set of symptoms & observable behaviors required for diagnosis by DSM-V criteria. They are categorical diagnoses that do not tell you the etiology, i.e., the genetic or other medical cause of characteristic symptoms in a individual. 40-70% of people with Autistic Spectrum Disorder also have ADHD. Neuropsychological testing will help determine if he has both.