Increased Excretion of Mast Cell Mediator Metabolites During Mast Cell Activation Syndrome

One requirement for diagnosing mast cell activation syndrome (MCAS) is an increase, above an established baseline level, in serum tryptase by 20% plus 2 ng/mL. However, there is no consensus of what constitutes excretion of a substantial increase in metabolites from prostaglandin D₂, histamine, or leukotriene E₄ in MCAS.

Ratios of acute/baseline levels for each urinary metabolite that accompanied tryptase increases of 20% plus 2 ng/mL were determined.

Mayo Clinic databases of patients with systemic mastocytosis with or without MCAS were reviewed. Patients with the requisite increase in serum tryptase during MCAS were examined for those who also had acute/baseline measurements of urinary mediator metabolite(s).

Ratios of acute/baseline levels for tryptase and for each urinary metabolite were calculated. For all patients, the average acute/baseline ratio (SD) for tryptase was 4.88 (3.77). Average ratios of urinary mediator metabolites were: leukotriene E₄: 35.98 (50.59), 2,3-dinor-11β-prostaglandin F2α: 7.28 (6.89), and N-methyl histamine: 3.2 (2.31). The lowest acute-baseline ratios for each of the three metabolites accompanying a tryptase increase of 20% plus 2 ng/mL were similar, with values of about 1.3.

To the author's knowledge, this is the largest series of mast cell mediator metabolite measurements during episodes of MCAS that were verified by the requisite tryptase increase above baseline. Unexpectedly, leukotriene E₄ showed the greatest average increase. Acute/baseline increase of 1.3 or greater in any of these mediators could be useful for corroborating a diagnosis of MCAS.