One of my docs always tests neurotransmitters in saliva (saliva has to be taken 4 times a day, to get a whole day-profile for e.g. cortisole). What is found in the blood is mostly bound to proteins and therefore not biologically active. In saliva, you can test the active form. But I don't know if this is scientifically accepted. I know at least 2 labs which can do the saliva analysis in Germany, if that would help you.