Blood Test for Depression

I was reading that someone has developed a blood test for depression.

Bullshit.

We currently use a behavioural model of diagnosing mental illness. You have a mental illness if you exhibit a certain set of behaviours. There are many reasons why a person can exhibit a set of behaviours. An example I read recently is that while autistic people tend to display less behavioural signs of pain when getting a needle, recording physiological signs of pain shows a greater response than a normal person. Different people are different and they behave in different ways in the same circumstances.

Depression is often associated with low serotonin. Or at least they try to increase your serotonin to try to deal with depression. But two people with lower-than-average serotonin levels are not going to behave in the same way. A blood test that showed serotonin levels, for instance, would not tell you if a person behaved as a depressed person behaves.

There are problems with the behavioural model of mental illness. One of the primary ones is that we are probably conflating a lot of different problems because people with them act in similar ways. We might also be missing connections because people with similar underlying problems do not behave in similar ways. Having some kind of objective look at what is going on behind the behaviours is a useful step in finding out more.

But "depression" is a way people behave. If there is a blood test that has a high correlation with depression and that allows us to divide depression into two types - blood test positive depression and blood test negative depression - then that would tell us that the blood test shows a condition that gives rise to depression.

Imagine we had blood test for common viruses and bacteria that upset people's digestive systems. We would not call that a blood test for stomach aches.

What we don't and can't possibly have, is a blood test that tells us how people behave or how they feel. That's more complicated than what is in your blood - if it wasn't we wouldn't bother having these brain-things in our heads. What we definitely can't have is a blood test that, if it comes back negative, proves a person is not depressed and therefore should not, for example, receive insurance benefits.

And that's important because there are people with a lot of money out there who would like to give less money to people who have been giving them money so that they can keep more money for themselves, and to them, a "blood test for depression" must smell like an opportunity.