What labs should be ordered to diagnose mast cell activation syndrome?

Laboratory Testing for Mast Cell Activation Syndrome

Order serum tryptase (baseline and acute), 24-hour urine N-methylhistamine, leukotriene E4, and 11β-prostaglandin F2α to diagnose MCAS, with bone marrow biopsy reserved for patients with persistently elevated baseline tryptase >20 ng/mL or other concerning features. 1, 2

Essential Mast Cell Mediator Testing

Serum Tryptase (Primary Biomarker)

• Obtain baseline serum tryptase when the patient is asymptomatic to establish their personal reference value 1, 2
• Collect acute serum tryptase 1-4 hours after symptom onset during a suspected mast cell activation episode 1, 2
• Diagnostic threshold: An increase of ≥20% above baseline PLUS an absolute increase of ≥2 ng/mL confirms mast cell activation 1, 2
• Comparing acute to baseline levels is optimal for tryptase as a preformed mediator 1

24-Hour Urine Collection (Complementary Testing)

• N-methylhistamine (histamine metabolite): More reliable than direct histamine measurement, which is not recommended 1, 3
• Leukotriene E4 (LTE4): Peaks in 0-6 hour collections after episodes; can guide leukotriene antagonist therapy 1, 2, 3
• 11β-prostaglandin F2α (11β-PGF2α): Peaks in 0-3 hour collections; correlates with anaphylactic severity 1, 2, 3
• These urine tests are non-invasive, can be collected at home, and provide objective evidence when acute serum tryptase is difficult to obtain 3

Genetic and Clonality Testing

Peripheral Blood Testing

• KIT D816V mutation using highly sensitive allele-specific oligonucleotide quantitative PCR (ASO-qPCR) to identify clonal MCAS 1
• TPSAB1 α-tryptase copy number variation (CNV) via buccal swab to diagnose hereditary α-tryptasemia 1
• These tests distinguish primary MCAS (with somatic or germline mutations) from MCAS without known mutations 1

Bone Marrow Evaluation (Selective Indications)

When to Perform Bone Marrow Biopsy/Aspirate

• Baseline serum tryptase persistently >20 ng/mL 1, 2
• Adult-onset mastocytosis in the skin 2
• Abnormal blood counts or organomegaly 2
• Positive REMA or NIH scores with negative peripheral blood KIT mutation (due to low allelic burden) 1

Bone Marrow Analysis Components

• Aspirate: Assess percentage and morphology of neoplastic mast cells; review for associated hematologic neoplasm 1
• Core biopsy: Evaluate for multifocal dense mast cell infiltrates (major diagnostic criterion for systemic mastocytosis) 1
• Immunohistochemistry: CD117, CD25, tryptase (CD30 optional) to quantify mast cell burden and identify aberrant expression 1
• Flow cytometry: CD117, CD25, CD2 as standard markers for rare event analysis of neoplastic mast cells 1
• KIT D816V mutation testing on bone marrow if peripheral blood is negative, using highly sensitive assays (>80% detection rate in SM) 1
• Chromosome analysis and myeloid mutation panel to identify associated hematologic neoplasms 1

Important caveat: Bone marrow biopsy cannot identify mast cell activation itself—it only identifies clonality and systemic mastocytosis 1

Tests NOT Recommended

• Plasma or urine histamine levels: Use N-methylhistamine instead 1
• Heparin: Not validated as a marker of mast cell activation 1
• Chromogranin A: Resides in neuroendocrine cells, not mast cells 1

Practical Testing Algorithm

1. Initial workup during symptomatic episodes:

   • Serum tryptase (acute + baseline comparison) 1, 2
   • 24-hour urine collection for N-methylhistamine, LTE4, and 11β-PGF2α 1, 2, 3

2. Clonality assessment:

   • Peripheral blood KIT D816V mutation (ASO-qPCR) 1
   • Buccal swab for TPSAB1 α-tryptase CNV 1

3. Bone marrow evaluation only if:

   • Baseline tryptase >20 ng/mL persistently 1, 2
   • Clinical features suggesting systemic mastocytosis 1, 2
   • Peripheral blood KIT mutation negative but high clinical suspicion 1

Critical point: Diagnosis requires documented elevation of at least one validated mediator during symptomatic episodes affecting ≥2 organ systems, plus response to mast cell-targeted therapy 1, 2