Blood Tests Can Accurately Diagnose Depression

>"See, there is something physically wrong with you, and you need treatment."

...could be a pretty stigmatising way to talk about mental health problems. I know you're not saying that people with minor depression are "making it up"; but I feel the need to say that an illness doesn't need to be 'physical' to be real.

Sorry for the slight de-rail.

Unless I'm misreading what you're saying, I think you have Bayes's rule confused in that statement. If 75% of positive results are false, that's actually right in line with what you might want for a first screening.

Of course, you want to know not just P(X|Y), but also P(Y|X) -- you really need the joint distribution to determine how useful the test is.

For a rapid screening, you generally want a low false negative, and you're willing to compromise by having a high false positive rate as a result. The real thing you care about is P(test positive | have MDD), not P(have MDD | test positive). You don't care about the latter (within reason - obviously you could just have everyone test positive, but you're also trying to maximize the former probability subject to some constraint - usually time/money).

Psychiatric conditions are notoriously difficult to test/evaluate/treat because they are oftentimes heterogeneous, so I don't know what kind of numbers we should be looking for in this case, but I just wanted to clarify that having a majority of people who test positive ending up being healthy actually doesn't necessarily rule a test out as being a useful rapid-response screening, in the general case.

With no evidence presented you dismiss the entire body of scientific evidence which backs the rather basic idea that depression, like any other biological process can be caused by a wide variety of sources not just external factors.

Being very sad because of an internal reason and clinical depression are not the same thing though to an outsider they may appear the same. Much like being overweight because you eat too much and sit all day isn't the same as being overweight because your thyroid isn't functioning properly aren't the same thing. What you're doing here is the intellectual and moral equivalent of blaming people with an underactive thyroid for their weight gain because you can't tell the difference by looking at them.

You're right on one point depression is the symptom not the disease and there's huge room for misdiagnosis here. Likely our understanding will ultimately split clinical depression into many different diseases that cause it as we learn more but its simply foolish to dismiss the disease possibility out of hand.

I don't know how much experience you've had with actual clinical depression, but as far as I've seen, those people are not "more rational", and leaving them to their own devices makes things worse, not better.

Instead, I'll just post and call you a fucking idiot.

You know how I can tell you don't have depression?

This sounds to me like you took something wildly out of context. For example, is a depressed person more rational than someone who just won a hand of Blackjack? Probably. Is a depressed person more rational than the same person when not depressed? As someone who is currently helping a friend through a very bad bout of depression, I can say with confidence that the answer is no.

Doctors don't even know why these drugs work.

I know a lot of programmers who don't know how a transistor works. Does this invalidate their work?