"So, in your process of diagnosis by exclusion, what does that tell you, Doctor?"

ADHD is purely a diagnosis by exclusion with no definitive physical test.

Diagnosis by exclusion tends to occur where scientific knowledge is scarce, specifically where the means to verify a diagnosis by an objective method is absent.

Perhaps the largest category of diagnosis by exclusion is seen among psychiatric disorders where the presence of physical or organic disease must be excluded as a pre-requisite for making a functional diagnosis.