system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is the level of serotonin measured in a depressed patient?

A common explanation on depression today is that the person suffering from it has low levels of serotonin. But how is that measured and how do the doctor know what level of serotonin is normal in each individual?
gpt
They don’t measure it for depressed people in a clinical setting- I’m not sure if it would even be possible. The idea that depression is caused by chemical imbalances actually evolved from the fact that meds that affect the concentration of those chemicals can also act as anti-depressants. Basically we accidentally found some meds that work and came up with an explanation for why. 

Depression is complicated and we don’t understand it.