Keep in mind that true MCAS is similar but different than what we as Long Haulers experience. It basically takes a bone marrow test to diagnose true MCAS. My labs were all normal but my allergist pegged it for me. Histamine Intolerance with Mast Cell involvement. Walking into a doctors office with an internet diagnosis in hand can backfire the Physician Ego. My Allergist is an exception to that rule thank goodness.