Table 1. Biomarkers indicating systemic severe MC activation in patients.

Biomarker	Specificity for MCs	Sensitivity in anaphylaxis	Commonly used in daily practice
Tryptase	++*	+**	++
Plasma histamine	+/–		+
Urinary histamine metabolites***	+/–	++	++
PGD2 metabolites****	+/–	++	+
Urinary cysLT levels	–/+	++	+/–
Heparin	+++	–/+*****	–
DAO	–******	++	–

^*Basophils express very low amounts of tryptase, but MCs are a primary and major source of the enzyme.

^**The relatively low sensitivity of tryptase qualifies as a biomarker of massive MC activation and thus as a criterion of MCAS.

^****Relevant 24-h urinary histamine metabolites include N-methylhistamine and N-methylimidazoleacetic acid.

^*****Among PGD2 metabolites, the most commonly measured substance is urinary 11β-prostaglandinF2α.

^*****An increase in heparin is usually not measurable during an anaphylactic episode, unless the burden of MCs is very high (like in MC leukemia).

^******So far it is not known whether the increased DAO levels measured in patients during anaphylaxis are derived from MCs or (also) other cell types.

cysLT, cysteinyl leukotriene; DAO, diamino-oxidase; MCAS, mast cell activation syndrome; PGD2, prostaglandin D2; MC, mast cell.