Inclusion Criteria:  Presenting with at least one of the three criteria below as evidence of systemic mast cell activation:  Involvement of 2 or more organ systems, characterized by skin (pruritus, urticaria, flushing and angioedema), cardiovascular (tachycardia, syncope, and hypotension), gastrointestinal (diarrhea, nausea, vomiting, and gastrointestinal cramping) or respiratory/naso-ocular (wheezing, conjunctival injection, and nasal stuffiness) AND serum basal tryptase levels ≥8 ng/ml. One of the organ systems must be the cardiovascular system. Severe, recurrent anaphylaxis (Ring and Messmer grading ≥II) due to Hymenoptera sting, regardless of serum basal tryptase levels. Severe, recurrent anaphylaxis (Ring and Messmer grading ≥II), with cardiovascular involvement and documented event-related tryptase elevation fitting the formula 20% of baseline plus 2 ng/ml evaluated in at least 1 event.  Exclusion Criteria:  Patient previously diagnosed with any of the following WHO systemic mastocytosis (SM) sub-classifications: cutaneous mastocytosis only, indolent SM, smoldering SM, SM with associated hematological neoplasm, aggressive SM, mast cell leukemia, mast cell sarcoma Patient diagnosed with other myelo- or lympho-proliferative diseases, e.g., myelodysplastic syndrome, myeloproliferative neoplasm, etc.