Aye, I've just finished The Inflamed Mind A radical new approach to depression
by Edward Bullmore
https://www.amazon.co.uk/Inflamed-Mi.../dp/1780723504
He argues quite clearly that as there is no actual way to measure serotonin in the brain, we don't actually know how or why Prozac et al are offering relief - if you can't measure "low" serotonin pre medication, how can you measure serotonin post medication. The only measure is asking the patient, so how do you feel?
In the book he's more positive about blood tests, specifically levels of
CRP (C-Reactive Protein)
Cytokines
Both are inflammation markers
The argument being - lowering your inflammation increases your
resilience to depressive symptoms/episodes/longevity