That's The Diagnosis? Really?

I see a doctor, a psychiatrist, every few months.  She diagnosed me with severe depression early on in my visits.  Now, she adjusts my meds based on my feelings, mood, energy, sleeping habits and what is going on in my life.  I've been doing this for four years now.
How though?  Psychiatrists diagnose you and give you prescriptions. They ask you A LOT questions abut family, home life, life history, health history, etc.  Sometimes they ask you to fill out worksheets with a quiz or a mood scale on it.  This is how they make a diagnosis. There is no blood test.  There are no X-rays.  No head scans, at least for most patients.  Because the diagnoses for these diseases are based mostly on the patient's feelings, how does one know if the diagnosis is true?  I noticed how the doctor "rules out" other mental illnesses by asking symptoms, recent or past mood swings, or any actions or thoughts out of the ordinary.  There are no PHYSICAL tests for a mental illness.  Just a ruling out and ruling in based on symptoms.  It almost seems just a little unscientific being there is no physical test for it.  But, it is a diagnosed and real disease that some people still don't believe in. Because one's head is being diagnosed does not mean the illness is all in one's mind.

My husband told me he was having tremors in his right hand and some in his right leg.  He made a doctor's appointment because of it.  His primary doctor sent him for an MRI based in his symptoms.  He was informed the MRI was clear - no tumors, no abnormalities.  We both breathed a sigh of relief.  However, his doctor wanted him to follow up with a neurologist because the symptoms were ongoing and it was not clear why the tremors were happening.  My husband also had pain in his right shoulder and he has never had a good sense of smell.  Off to the neurologist we went.
The exam went well, at least that is what I thought until the end.  I sat in on his visit and all the muscle exercises he made him do seemed to be fine.  Then, he asked him to walk down the hall.  He noticed my husbands right arm was not swinging when he walked.  This is a sign of Parkinson's.  Five minutes later we heard his diagnosis, Parkinson's.  We were both shocked.  I immediately protested and said, "But the MRI was clear!!" The doctor then informed us that Parkinson's can't be detected from an MRI, a blood test, and X-ray or anything except for a muscle test and ruling everything else out.  It's a chemical imbalance in the brain.  I never knew this before.  I just thought you could detect it through brain scans and/or blood tests.  Well, needless to say we were stunned for quite a while after that.  It was not what we hoped.  As I learned quickly, Parkinson's is also diagnosed in an almost unscientific way.

So here we were, my husband and I diagnosed over a few years with chemical imbalances in our brains that can't be diagnosed the way most people think a disease needs to be proved to be "real".  Who would have thought?  But the difference between our diagnoses is that one is a familiar disease a major celebrity and many others have had for years.  People know it is a serious disease.  Well, the other is a disease celebrities and many others have had for years but don't like to talk about, but it is a serious disease and it still carries a stigma.