DEPRESSION
Unfortunately there is no brain scan or blood test that can be used to diagnose when a person has a depressive illness. The diagnosis can only be made from the symptoms. Generally speaking a diagnosis of depression will be made if a person has a persistently low mood that significantly influences their everyday life and has been present for two weeks or more, and there are also three or four or more other symptoms of depression.
Doctors make a diagnosis of depression after assessing the severity of the low mood, other associated symptoms and the duration of the problem.