Mast Cell Activation Syndrome: Recognition and Management

Diagnostic Criteria

The American Academy of Allergy, Asthma, and Immunology recommends that Mast Cell Activation Syndrome (MCAS) requires three mandatory diagnostic criteria to be met simultaneously: recurrent symptoms affecting at least 2 organ systems concurrently, documented elevation of mast cell mediators during symptomatic episodes, and clinical improvement with mast cell-targeted therapies 1, 2, 3
MCAS is characterized by episodic and recurrent symptoms, not persistent or chronic, with symptoms affecting multiple organ systems such as cardiovascular, respiratory, dermatologic, gastrointestinal, or neurologic 2

The National Comprehensive Cancer Network recommends classifying patients into three categories based on underlying pathology: Primary MCAS (Clonal), Secondary MCAS (Reactive), and Idiopathic MCAS 4, 5, 1
Primary MCAS is characterized by the presence of a KIT D816V mutation, which may progress to systemic mastocytosis, although this is uncommon 1, 4
Secondary MCAS is triggered by underlying IgE-mediated allergies, chronic inflammatory disorders, or neoplastic conditions, with no KIT mutations present 4, 5

The American Academy of Allergy, Asthma, and Immunology recommends obtaining baseline serum tryptase when completely asymptomatic to establish a personal reference value, and collecting acute serum tryptase 1-4 hours after symptom onset during episodes 2, 6, 3
The diagnostic threshold for serum tryptase is a ≥20% increase above baseline PLUS an absolute increase ≥2 ng/mL 4, 2, 6

The National Comprehensive Cancer Network recommends starting with combined H1 and H2 receptor antagonists at 2-4 times standard FDA-approved doses as first-line therapy for all patients 7, 2, 8
Second-line therapy includes mast cell stabilizers such as cromolyn sodium, and mediator-specific therapy such as leukotriene antagonists and aspirin 7, 2, 8

The American Academy of Allergy, Asthma, and Immunology recommends that all patients carry 2 epinephrine auto-injectors and immediately assume a supine position in the event of anaphylaxis 7, 8
Administer intramuscular epinephrine 0.3-0.5 mg and call emergency services, with supplemental oxygen and IV fluids as needed 8

The American Academy of Allergy, Asthma, and Immunology reports that patients with idiopathic MCAS followed for >15 years did not develop mastocytosis, and patients with indolent systemic mastocytosis have normal life expectancy similar to the age-matched general population 1, 4
Treatment is symptom-based and typically lifelong, with adjustments based on mediator profiles and clinical response 1