Earlier quoted context omitted.
I know about the problems around the DSM. That doesn't make ADHD less of a "thing" though. Turns out that a questionnaire can diagnose ADHD. This diagnosis predicts anatomical, physiological and psychological differences, and on top of that certain negative consequences and suffering on the part of the patient. Treatment reduces that suffering. What more do you need?
Questionnaires are cheap but less reliable than brain (PET, fMRI) scanning or quantitative attentional testing. What is the impact of a false positive diagnosis on a still-developing young brain? Adults are a different story - they can make their own decisions.
I mentioned questionnaires as a means of reaching a diagnosis which is not dependent on instrumental testing, to make a point about the consistency of subjective evaluation, objective findings, suffering and treatment.
But in General, ADHD diagnosis is not based on PET, fMRI or EEG.