[joesmo]

"Blood levels of serotonin are measurable -- and have been shown to be lower in people who suffer from depression – but researchers don't know if blood levels reflect the brain's level of serotonin.

"Also, researchers don't know whether the dip in serotonin causes the depression, or the depression causes serotonin levels to drop.

"Although it is widely believed that a serotonin deficiency plays a role in depression, there is no way to measure its levels in the living brain." -- (http://www.webmd.com/depression/features/serotonin)