Diagnosis Criteria

Criteria defining Mast Cell Activation Syndrome (MCAS)

Major criteria

  1. Constellation of clinical complaints attributable to pathologically increased MC activity (MC mediator release syndrome)

Minor criteria

  1. Multifocal or disseminated infiltrates of MCs in marrow and/or extracutaneous organ(s) (e.g., gastrointestinal or genitourinary tract; >19 MCs/high power field)
  2. Abnormal spindle-shaped morphology in >25% of MCs in marrow or other extracutaneous organ(s)
  3. Abnormal MC expression of CD2 and/or CD25 (i.e., co-expression of CD117/CD25 or CD117/CD2)
  4. MC genetic changes (e.g., activating KIT codon 419, 509 or 560 mutations) shown to increase MC activity
  5. Evidence (typically from body fluids such as whole blood, serum, plasma, or urine) of above-normal levels of MC mediators including:
    • tryptase
    • histamine or its metabolites (e.g., N-methylhistamine)
    • heparin
    • chromogranin A (note potential confounders of cardiac or renal failure, neuroendocrine tumors, or recent proton pump inhibitor use)
    • other relatively MC-specific mediators (e.g., eicosanoids including prostaglandin (PG) D2, its metabolite 11-β-PGF2α, or leukotriene E4)
  6. Symptomatic response to inhibitors of MC activation or MC mediator production or action

Criteria proposed to define mast cell (MC) activation syndrome when all other diagnoses that could better explain the full range and chronicity of the findings in the case have been excluded (modified from [1]). The diagnosis mast cell activation syndrome is made upon fulfilment of the major criterion plus at least one minor criterion.


  1. Molderings GJ, Brettner S, Homann J et al. (2011) Mast cell activation disease: a concise practical guide for diagnostic workup and therapeutic options. J Hematol Oncol 4:10