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