When there are large amounts of the amino acid, tryptophan, it crosses at higher concentrations
and more
serotonin is subsequently made. When the blood
levels are tested - I don't know exactly what that means. One would think it would be reflective of the concentration in the brain. I've been on SSRIs for a long time, theoretically preventing unused serotonin from being reabsorbed back into the nerve fiber from which it was released.