"No biological tests confirm major depression."
Interesting point, but actually, you can say that for a lot of things in medicine. And I think it's important to remember that the concept of "health" doesn't (and shouldn't) consist of solely "biological" factors.
In fact, some of the patients who I regard to be very "unhealthy" consider themselves to be very healthy thank you. (And
vice versa).
As one who has initiated, and monitored, drug therapy for depression, many times, I have to admit, it can sometimes be a difficult call. We try and make the process scientific, but it isn't always easy.
I suspect many prescribers turn to the pad, when actually they'd rather be able to prescribe 2 weeks surfing, a new boss, someone to talk to, or someone to look after the kids once in a while.
Funny thing mental illness- as common as asthma, (which is also difficult to prove through biological tests), yet still remains stigmatised and not well understood by people.
Walk down the road with one leg (!) and people sympathise, having mental health problems is less understood.