Is the serotonin level in the blood corelated with the level in brain?

[matthewmate]

Is the serotonin level in the blood corelated with the level in brain?
If yes, why doctor before ssri drugs apply, do not ussually test serotonin level in blood?

[Paxiled]

Serotonin is found all over the body, mostly not in the brain.  Serotonin is also not the cause of mental illness, playing with it just sometimes makes us feel better but it doesn't cure anything.  There are people with low serotonin levels, which would involve a problem with the body's ability to manufacture it from tryptophan and B6 and other co-factors that are taken in when you eat, but that doesn't correlate with mental illness.  Also, the drugs we use don't create more serotonin in the body, they alter the way the body breaks it down by blocking it and directs the serotonin to a couple of targeted receptors (which ends up with the body closing down other receptors as they're no longer considered needed).  But you're still working with the same amount of serotonin your body was manufacturing all along, it's just utilized in a way that allows it to wash longer in the targeted receptors instead of being regularly broken down and replaced with fresh serotonin.  Meaning, it wouldn't make any clinical difference for you as it would still be trial and error as to whether any particular drug would do anything for you or not.