I've been studying the website of Alison Vickery, an Australian Certified Health Coach whose practice involves helping patients with histamine/mast cell issues and chronic fatigue syndrome. http://alisonvickery.com.au/living-health-clinic/ Alison provides a list of supplements, two of which I've tried only briefly (quercetin and DAO). So far, I haven't been able to find a medical practitioner who can help determine whether I have a DAO deficiency (in which case quercetin is unlikely to be helpful), or mast cell activation (in which case a DAO enzyme is unlikely to be helpful). Here's the segment of her article I'm wondering about: "Cytochrome P450 (CYP450) is a group of enzymes that convert substances, including medications and supplements. There is a high degree of variability in these enzymes. Only 50% of the population has a “normal” enzyme. It is important to know your own personal CYP450 profile. 23andMe® does not provide this profile. Some substances increase the function, some reduce the function, and some have no effect whatsoever. One of the primary reasons that people experience adverse reactions to medications and supplements is if they are a “poor metaboliser” of the CYP450 enzyme used to convert the supplement. For example, if you are a poor metaboliser of CYP2D6 then both quercetin and luteolin may be toxic to you whereas Pycnogenol®, which is not metabolised by CYP2D6, will not." I've looked into CYP450 testing, and it appears to be available in Canada for $1045 (Ouch!). http://www.pharmacy.ca/ptmain.shtml Cyrex Labs also includes CYP450 testing in its Array 5, although it doesn't specify CYP2D6 which appears to be involved in metabolizing both quercetin and luteolin. I'm not a scientist, and won't even pretend to know what all this means. https://www.cyrexlabs.com/CyrexTestsArrays/tabid/136/Default.aspx I'm just wondering . . . has anyone with mast cell/histamine issues had CYP450 testing, and if so, was it helpful?