One last psychiatry rant
There will never be such a thing as a biologic marker for depression. Even if it existed, of what use would it be? If I felt fine but had some biological marker that indicated I had depression, should I take medication? Why? What would be the point? Likewise, if someone feels horribly depressed but doesn't have the marker, should we not treat them? Of course not.
I guess you could use it for screening, but what's the point of screening for depression? The point of screening is to treat illness before it becomes symptomatic. But depression, by definition, cannot be asymptomatic. So, why screen for it?
There may be other conditions out there (i.e. hypothyroid) that secondarily cause depression. And there may very well be biological markers to test for these diseases. But for a biological process to cause no other physical symptoms except for those associated with depression? I suppose it's plausible but, I think, highly unlikely.
How do the patients in these studies get their diagnoses anyway? The DSM. Diagnosing psychiatric illness isn't at all like diagnosing hypertension, anemia or diabetes where a simple number gives you the diagnosis. It requires people to make a judgment call, in every case, as to whether the DSM criteria apply. And how often do psychiatrists agree on whether the DSM criteria apply? Depression is one of the most reliable DSM diagnoses and it is only about 80% reliable. Psychiatrists agree on whether or not a given individual is depressed about 80% of the time. And unlike diabetes or hypertension where a diagnosis is either correct or incorrect there is no higher authority to say who made the "right" psychiatric diagnosis. Therefore, since the diagnosis itself is only 80% reliable, any biological marker we found could only be at most 80% reliable.
Ok, let's say that you believe the DSM only approximates these "biological" illness and you believe that with biology and genetics we will finally be able to make the "true" diagnosis. The DSM becomes obsolete. Schizophrenia, for example, is found to be a heterogeneous group of illnesses each with it's own abnormal proteins produced by their own abnormal genes. Schizophrenia is no longer a psychiatric diagnosis but a neurological diagnosis. I once had a professor who said that one day, there will no longer be any psychiatrists but only psychologists and neurologists. I agree. But, while neurology might grab a few, I think psychology is going remain in control of the vast majority of current psychiatric diagnoses.