Eli5: why is there no test for the “chemical imbalance” that is often mentioned for depression?
In: Chemistry
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
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