Background: Mast cell activation syndrome (MCAS), a recently recognized nonneoplastic mast cell disease driving chronic multisystem inflammation and allergy, appears prevalent and thus important. We report the first systematic characterization of a large MCAS population.

Method: Demographics, comorbidities, symptoms, family histories, physical examination and laboratory findings were reviewed in 298 retrospective and 115 prospective patients with MCAS. Blood samples from prospective subjects were examined by flow cytometry for clonal mast cell disease and tested for cytokines potentially driving the monocytosis frequent in MCAS.

Results: Demographically, white females dominated. Median ages at symptom onset and diagnosis were 9 and 49 years, respectively (range: 0-88 and 16-92, respectively) and median time from symptom onset to diagnosis was 30 years (range: 1-85). Median numbers of comorbidities, symptoms, and family medical issues were 11, 20, and 4, respectively (range: 1-66, 2-84, and 0-33, respectively). Gastroesophageal reflux, fatigue and dermatographism were the most common comorbidity, symptom and examination finding. Abnormalities in routine laboratories were common and diverse but typically modest. The most useful diagnostic markers were heparin, prostaglandin D₂, histamine and chromogranin A. Flow cytometric and cytokine assessments were unhelpful.

Conclusions: Our study highlights MCAS׳s morbidity burden and challenging heterogeneity. Recognition is important given good survival and treatment prospects.

Authorship and Conflict-of-Interest Statements

Dr. Afrin was the principal investigator and author and declares no conflicts of interest.

Dr. Self performed and interpreted the flow cytometry studies, reviewed the manuscript, and declares no conflicts of interest.

Mr. Menk performed statistical analyses, reviewed the manuscript, and declares no conflicts of interest.

Dr. Lazarchick performed and interpreted the cytokine analyses, reviewed the manuscript, and declares no conflicts of interest.

• UL1 TR000114/TR/NCATS NIH HHS/United States