Eli5: why is there no test for the “chemical imbalance” that is often mentioned for depression?
In: Chemistry
Because they don’t really know how it works, they think they have an idea, but they don’t really know. In fact, they backtrack all the time all over the board on a whole range of issues because every now and then, some realize, they knew wrong, so they can still (know wrong). Just keep that in mind.
Depression is as much a software issue as it is a hardware issue. The “chemical imbalance” theory implies it’s just a hardware issue. But it’s more than that.
You can measure concentrations of chemicals in the body. But you can’t measure what someone is thinking with a lab test.
I think a major point here is that it’s a lot of guesswork treating depression and other mental issues. This is seriously a huge gray area with a lot of unknowns. Hard to test for something when you aren’t even sure how it works. Some will say mechanism unknown. About all they know sometimes is that it works for some people. Testing on humans is hard.
So usually tests are done in blood since that’s easily accessible. The chemical imbalance that occurs in mental illness is a chemical imbalance of neurotransmitters. Basically those chemicals aren’t available in your blood, but inside your brain, and even in “normal” brains, they get reabsorbed or degraded fairly fast, so you couldn’t even really measure it in cerebrospinal fluid, since the chemicals wouldn’t be widely available there. Only way to test it would maybe doing a biopsy of a fairly large chunk of the brain and test the presence or absence of said chemicals, but for obvious reasons, that wouldn’t be a procedure that’d be useful at all. There are some tests that have been used before to sorta look at the somatic, biological side of the equation. MRI can be used to measure blood flow to certain areas of the brain, and if some areas are less or more active than others you can sorta deduce it’s degree of chemical activity, but this type of test isn’t done usually because a) it’s super expensive b) even though treating depression is hard, diagnosing it is fairly easy, and can be done in just a couple of sessions of talking to your patient, and c) said changes usually occur in very subtle ways, in quite severe depression, so at that point it has little diagnostic use. It has been used in research though, but fairly useless in clinical practice
“Chemical imbalance” is an unproven theory that is embraced because it sounds nice. Much better to say, “I’m depressed because of a chemical imbalance in my brain” than, “I’m depressed because I’ve made a mess of my life.”
If we want people to seek treatment, we have to remove the shame.
You’d probably need a pre- disorder reading of a test to be able to compare changes from their baseline but most depressed people don’t go to the doctor months before they feel unwell to get a baseline test just in case.
It’s because the chemical balance hypothesis was something that was devised by pharmaceutical companies as a marketing ploy, and has never been substantiated by any empirical evidence.
https://www.theguardian.com/society/2018/jan/07/is-everything-you-think-you-know-about-depression-wrong-johann-hari-lost-connections
Even psychiatry is admitting to this, these days: https://www.psychiatrictimes.com/view/debunking-two-chemical-imbalance-myths-again
Because it’s bs. 99% of treated depression is not for medical reasons, it’s because the world is shit, or specifically that person’s life is more shitty than normal.
I wouldn’t explain it like this to a five year old though.
Neuroscientist here: Because the “chemical imbalance” theory is bullshit. It was a real theory several decades ago, which was abandoned in the field because it doesn’t make sense / doesn’t work. But it is simple and easy to explain (despite being false) and therefore continues to propagate.
To be clear, it’s not that it’s a simplification. It’s just wrong.
If it WERE a chemical imbalance, it would be something you coughs test for, you are right about that, although the test would probably not be a blood test, might require a spinal tap or PET scan (best ways to measure specific chemicals in brain). It would also be simple to treat.
Pretty much ANY neat, simple story that someone tells you about the brain is usually false.
There are several parts to an answer:
1) As somebody already mentioned. (Major) Depression is actually “the symptom”. The underlying factors causing depression are varied and cannot be reduced to a single marker, as for example with diabetes. In short, the physiological mechanisms of depression (what is malfunctioning in the brain) are not fully understood and there is a lot of active research going on.
2) The chemical imbalance that you mention refers to neurotransmitters in specific parts of the brain, meaning, you would have to test the levels of such neurotransmitter directly in those areas, since a blood test would not be enough for that. And as I mentioned, even if you test this, this would only work for a very narrow group of patients, since the physiological causes of depression are very varied.
3) Not part of the answer, but if you google “depression biomarkers” you’ll find recent research regarding the kind of tests that could allow this. Which would be helpful for, for example, knowing what kind of depression the patient has.
TLDR It is possible to test this, but its a bit more complex than a blood test
Sources:
(1) I did neuroscience research for many years, including depression
(2) https://www.sciencedirect.com/science/article/abs/pii/S0166432817318521
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