Is there a device that measures serotonin simply by pricking blood into it?

I'm not positive but I don't think so. There is a Kit (quite expensive) that can be used to tell blood levels of serotonin: The Serotonin-ELISA kit provides materials for the quantitative measurement of chemically derivatized serotonin in serum, plasma, urine, tissue homogenates and tissue culture supernates. The sample preparation, i.e. derivatization of serotonin to N-acylserotonin, is part of the sample dilution and is achieved by incubation of the respective sample with the "Acylation Reagent". But knowing Serotonin levels in the brain are not so easy, since Serotonin is restricted by the Blood-Brain barrier. Spinal Tap for this I believe. TommyTrouble