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

Question

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

In: Chemistry

Answer 1

Neurotransmitter release not only varies in amount, but in timing, as bursts. The mix of these neurotransmitters differs in different parts of the brain and in response to different situations/actions.

So even if you COULD directly measure it, you’d have to do so over time, in various areas, in exposure to various different stimuli/situations.

And even if you could do THAT, it wouldn’t be helpful since epigenetics modifies the expression of genes in response to stress, making every human brain different.