I wanted to update this. I saw a specialist who deals with MCAS too, and he ran a couple of blood tests, not the complex 24 hour urine tests, and offered me Cetirizine and Ranitidine to see whether it would improve my condition. I saw him a while ago but have been too busy to update. I still haven't filled the Rx, and don't think I will. I may see him again. He didn't see particularly pushy at all about taking the meds and said he'd be fine if I didn't, but I'm not sure what else he can do if I don't take the meds, since one of the 3 key methods of seeing whether one has MCAS is empiric, which required medication and observation. Testing for markers is imperfect and often fails, and in Canada many of the key markers are not tested as they are in the USA.

I have a followup appointment in May or June, can't recall, but I may push it back or call and see whether he wants to see me if I haven't taken the meds. I took Zantac for years, so Ranitidine, and I think that was a huge mistake. I don't intend on starting it again.