Serum Tryptase

Serum tryptase is the most commonly ordered lab test for mast cell disorders, but its clinical utility for MCAS specifically is limited. For a broader view of diagnostic limitations and emerging alternatives, see Existing Diagnostics.

What It Measures

Tryptase is a protease released during Degranulation. Serum tryptase reflects the amount of tryptase circulating in the blood at the moment of the draw.

Baseline tryptase (drawn when the patient is not in an acute flare) reflects overall mast cell burden — how many mast cells exist. Persistently elevated baseline tryptase (>20 ng/mL) suggests mastocytosis (increased mast cell numbers) rather than MCAS.

Acute tryptase (drawn during or shortly after a symptomatic episode) reflects recent degranulation. The Consensus-2 diagnostic criteria for MCAS require a rise of ≥20% + 2 ng/mL above the individual’s baseline during a symptomatic episode.

The Problem for MCAS

In many MCAS patients, tryptase is normal — both at baseline and during flares. This is because:

• MCAS often involves piecemeal Degranulation (selective mediator release without full granule dump). Tryptase may not be released during piecemeal events.
• Tryptase has a relatively short half-life (~2 hours). If the blood draw doesn’t happen within 1-4 hours of an acute event, the tryptase may have already cleared.
• Chronic, low-grade activation produces symptoms from Histamine, Prostaglandins, and Cytokines without necessarily releasing enough tryptase to register above the reference range.

A normal tryptase does not rule out MCAS. This is a significant source of diagnostic frustration and is the central disagreement between the stricter and broader MCAS diagnostic criteria. The Mast Cell Reactivity Test project is exploring a fundamentally different approach — measuring mast cell reactivity directly ex vivo rather than trying to catch transient mediators in blood.

Hereditary Alpha-Tryptasemia (HαT)

A genetic condition (extra copies of the TPSAB1 gene) that produces constitutively elevated baseline tryptase without necessarily having mast cell disease. Prevalence is approximately 5-6% of the general population. Important to consider in the differential because elevated tryptase doesn’t automatically mean MCAS or mastocytosis.