I have had serious abdominal pain since childhood, urticarias, cough problems, and other symptoms that match the new international criteria for MCAS. I had to do my own online research to basically self-diagnosis. Over a life-time I have received about a half dozen or more misdiagnoses, including childhood abdominal pain so severe I fainted and had my appendix taken out only to learn it was normal. I had dozens of x-rays of abdomen, and nothing could be found, all negative so lived for another twenty plus years with diagnosis and treatment for a duodenal ulcer, though that never appeared in any x-ray. Then I got diagnosis of IBS, which seems to be correct but researchers now know that IBS and MCAS often are comorbid syndromes. I've been prescribed various medications for IBS none of which did much. Finally a gastroenterologist tested me a new diagnosis Small Intestine Bacterial Overgrowth (SIBO) which came back positive. That explained the IBS symptoms. The gastroenterologist recommended a Xifaxin trial. It is a very expensive drug with no evidence of effectiveness as far as I could find any studies. So I use probiotics and Ultimate Meal green food for every breakfast, and this helps the IBS symptoms. But not the MCAS symptoms. At least I got four boxes of free samples of Xifaxin, which I can use for international travel if I get traveler's 'revenge'. For the newly emerging chronic cough problem I went to an ENT and allergist and all tests came back negative. Four different doctors and specialists could not figure out a diagnosis. A couple years ago I managed to convince my Medicare family practice doc to get me one of the genetic tests for mastocytosis. (My grandfather died of what my father reported was an 'odd' form of leukemia; my guess is it was mastocytosis leukemia.) That came back negative. But that test is not one for MCAS. There seems to be no good test yet available for MCAS. Finally last summer when I was in Canada my allergy-like symptoms got much worse, the cough kept me from going public. I went to the local drop-in family health clinic, and the doctor suggested, as I described my symptoms and research findings that I do two trials - a powdered inhaler and if that failed, a generic of Singulair. The inhaler only caused my chronic cough to get extremely worse. The Singulair reduced the cough symptom about 60% in a few days. I read a 2014 medical research article on MCAS symptoms and positive response to an antileukotriene is now one of the diagnostic criteria for MCAS. I take a low dose daily and it has seemed to continue to help. Osteopathic neck manipulation has also helped with chronic cough reduction, as there seems to also be a structural problem in part may be due to my mild scoliosis.

Here is my own short summary of diagnostic criteria based on my reading of medical research articles, Akin, Valent and Metcalf (2010) revised with Petra et al (2014):
Table II. Criteria for the diagnosis of mast cell activation syndrome (MCAS).
Idiopathic MCAS.
After primary and secondary rule out: no detectable clonal MC, no reactive disease, and no allergen-specific IgE Episodic symptoms consistent with mast cell mediator release affecting two or more organ systems evidenced as follows:
• Skin: flushing, urticaria pigmentosa, pruritus (itching), angioedema, dermatographism; rashes, hives
• Respiratory: wheezing, sore throat, stridor; [cough]
[Idiopathic anaphylaxis, with urticaria (during anaphylaxis, unlike SM) and high IgE.]
• Cardiovascular: chest pain, hypotensive syncope or near syncope, tachycardia
• Gastrointestinal: abdominal pain (cramping, bloating), nausea, vomiting, diarrhea, malabsorption, esophagitis; gastroesophageal reflux
• Naso-ocular: pruritus, nasal stuffiness; conjunctival injection
• Neurologic: headache, memory and concentration difficulties/brain fog, paresthesia, peripheral neuropathy
• Musculoskeletal bone/muscle pain, degenerative disc disease, osteoporosis/osteopenia
• Systemic: anaphylaxis, fatigue, faintness
Documentation of an increase of a validated urinary or serum marker for MC activation: esp. increased tryptase
Response to anti-mediator therapy (decr in frequency or severity or resolution of symptoms: e.g., H1, H2, antileukotriene meds

I have or have had 15 of these symptoms. Response to the antileukotriene seems fairly definitive to me.
Wishing everyone in this thread best of luck in getting good diagnosis and finding optimal medication and diet.