I get to hear this question or it's variants every now and then.
Imagine the scenario in my consultation room when someone walks in quite agitated about something, speaks to me at length and then asks for my opinion. (The secret terminology for this process is called History and Mental Status Examination!)
I proceed to tell the noble soul that his condition fits a particular diagnosis. That is; if there is a diagnosis in the first place.
The diagnosis could be Major Depressive Disorder, Generalized Anxiety Disorder, Panic Disorder, Obsessive Compulsive Disorder or one of the types of Schizophrenia. And then there are so many other categories too...
Then I begin to talk about the treatment options. Medication and Psychotherapy in various combinations...
Sure enough; I am asked...
Is there any blood test that can prove Bipolar or Schizophrenia?
Do you suggest any CT scan or MRI scan of the brain.
My answer is - No !
Mark my words. There is absolutely NO standard diagnostic blood test yet for most psychiatric disorders.
Also; there is generally NO physical abnormality that a typical CT or MRI scan can pick up.
You say... So; then; doctor how did you diagnose my condition?
My answer would be that we psychiatrists base our diagnosis on "cluster of symptoms". Not just one symptom but a cluster of symptoms.
And I get my knowledge of your symptoms through my conversation with you. Through the observation of your behavior... As well as the details supplied by a reliable near and dear one.
Above all; I don't believe in unnecessary investigations. But then sometimes investigations are important to a psychiatrist.