Mast cell activation syndrome (MCAS) involves the skin, gastrointestinal, cardiovascular, respiratory, and neurologic systems, classified as primary, secondary, and idiopathic. Earlier criteria for MCAS diagnosis included episodic symptoms with mast cell mediators affecting two or more organ systems with urticaria, angioedema, flushing, nausea, vomiting, diarrhea, abdominal cramping, hypotensive syncope, tachycardia, wheezing, conjunctival injection, pruritus, nasal stuffiness, decrease in frequency, severity, or resolution of symptoms with anti-mediator therapy including H₁/H₂ receptor antagonists, anti-leukotrienes, or mast cell stabilizers. Laboratory data includes an increased validated urinary or serum markers of MCAS, documentation of an increase of the marker above the patient's baseline value during symptomatic periods on more than two occasions, or baseline serum tryptase levels that are persistently above 15 ng/mL. Laboratory data also includes an increase of the tryptase level above baseline value on one occasion. Other assays are 24-h urine histamine metabolites, PGD₂ or its metabolite, and 11-β-prostaglandin F₂ alpha. A recent global classification is a response of clinical symptoms, a substantial transient increase in serum total tryptase or increase in other mast cell-derived mediators, histamine or PGD2 or urinary metabolites, and agents that attenuate production or mast cell mediator activities. "Spectrum of MCAS disorders" has been proposed, highlighting symptoms' diagnostic tests and treatments.