The symptoms of depression and criteria are contained in the DSM. They don't involve brain chemistry or functioning (per se), e.g. "Recurrent thoughts of death

The symptoms of depression and criteria are contained in the DSM. They don't involve brain chemistry or functioning (per se), e.g. "Recurrent thoughts of death, recurrent suicidal ideation without a specific plan, or a suicide attempt or a specific plan for committing suicide." The underlying reasons for those symptoms may involve chemical imbalances, but those aren't established or tested for. They don't take a CSF sample or do an MRI for a diagnosis.