Table 5.

MCAS Phenotypes

Primary phenotype: (mono)clonal MCAS	Fulfills MCAS diagnostic criteria (Table 4) + the presence of (mono)clonal populations of MCs with the presence of activating mutations in KIT (usually KIT D816V) and/or aberrant expression of CD25
Secondary phenotype: nonclonal MCAS a	Fulfills MCAS diagnostic criteria (Table 4) + the presence of allergy, hypersensitivity, and/or other reactive condition; these conditions can be IgE-dependent or -independent: IgE-dependent secondary MCAS triggers: food, drugs, hymenoptera venom, inhalant allergens IgE-independent secondary MCAS triggers: endogenous peptides, drugs that interact with MRGPRX2 (vancomycin or narcotics), physical stimuli, stress, toxins, venoms, complement activation (C3a, C5a, cytokine receptors, MRGPRX2, viral receptors, toll-like receptors)
Idiopathic phenotype	Fulfills MCAS diagnostic criteria (Table 4) + no evidence of (mono)clonality + no evidence of allergy, hypersensitivity, and/or other reactive condition

IgE, immunoglobulin E; MC, mast cell; MCAS, mast cell activation syndrome.

^aMCs can be elevated in specific tissue sections from reactive expansion. Adapted from Gülen et al.¹⁷