Blood test for Depression?

A couple months ago there were a few articles that ran about developing a blood test for Major Depressive Disorder ( see: The Chicago Tribune, CBS News, TIME, Medscape, Huffington Post, Newsweek, New York Magazine and HealthDay).  This sounds great: it could revolutionize the diagnostic process making more precise distinctions between Depression and other disorders, syndromes and episodes.  However I’m just not sold on the technology yet, and one of the reasons is that frankly we just don’t know that much about Depression and other psychiatric disorders (or about the brain for that matter).  So we may be able to identify markers in the blood of “something” but we don’t necessarily know what that means and I fear this will give false confidence to those performing and receiving the tests.  You see, human behavior is just too complex to distill down lab values (for now, at least).  There are many people who receive antidepressant medications who really don’t need them, and there are many others who could benefit from medication or psychotherapy but don’t receive it; ideally such testing could improve much of this, but I worry that when a truly “depressed” person (layman’s use of “depressed,” not clinical/diagnostic) takes a blood test and his lab values are not indicative of Major Depressive Disorder (the clinical diagnosis), he may not be encouraged to seek the psychotherapy that could very well improve his quality of life, help him function at a higher level and engage in healthier behaviors.  Blood tests, and all other diagnostic tests for that matter, can be enormously useful when used in the context of clinical judgement, and can be enormously harmful when not.