ADHD seems to get diagnosed based on the DSM-IV. It's not binary. There are three types "ADHD, Combined Type", "ADHD, Predominantly Inattentive Type" and "ADHD, Predominantly Hyperactive-Impulsive Type". A DSM-IV diagnosis however doesn't give you are score.

There are huge issues with the DSM IV. In it's philosophy the DSM describes symptoms. As a result it's authors don't see the necessarity to back up their diagnosis with scientific evidence that get's cited in the DSM.

Psychopathy on the other hand gets diagnosed based on a specific PCL-R test that Robert D. Hare developed. The test is result of psychometric work. It's designed to predict recidivism and violence. The DSM-IV doesn't recognize psychopathy but instead uses the category of antisocial personality disorder.

In the case of depression you also have on the one hand the DSM-IV criteria and on the other hand the Hamilton Rating Scale for Depression.

Psychiatry studies that use tests that are optimized to predict something like the PCL-R should probably be trusted more than the DSM-IV. The present debate about the new DSM-V can also give you a good illustration of it's nature.