Like jimi, I am not aware that anything can be learned about one's mental state by measuring seotonin levels in the blood. Not that I know everything, but I've been treated for depression for decades and am always reading up on what doctors look at in treating this condition. I'ld be very interested to know what a doctor is hoping to discover by ordering tests of blood serotonin.

Serotonin does a lot of things throughout the body. I have read that it plays a role doing something in the digestive tract.

What has your doctor given as a reason for doing this test, and what are the results supposed to mean?

I'm not sure that changes in blood levels of serotonin correlates with changes in serotonin availability within the synaptic clefts between nerves, where serotonin levels are theorized to affect mood.