I've found someone offering plasma histamine and diamine oxidase (DAO) tests. I have not tried it myself and as such do not endorse them. Please let me know your experience if you have used them.
I'm the Low Histamine Chef! Please feel free to ask me any questions. Please visit the site for information on Histaminosis/Histamine Intolerance and to download a copy of the Diamine Oxidase Support Recipe Book today!