What laboratory tests should be done for a patient suspected of having Mast Cell Activation Syndrome (MCAS)?

Laboratory Testing for Suspected Mast Cell Activation Syndrome

For a patient suspected of having MCAS, obtain baseline serum tryptase when asymptomatic, acute serum tryptase 1-4 hours after symptom onset, and 24-hour urine collection for N-methylhistamine, leukotriene E4, and 11β-prostaglandin F2α, followed by genetic testing for KIT D816V mutation and TPSAB1 α-tryptase copy number variation. 1

Essential Mast Cell Mediator Testing

Serum Tryptase (Primary Diagnostic Marker)

• Collect baseline serum tryptase when the patient is completely asymptomatic to establish their personal reference value 1, 2
• During a suspected activation episode, collect acute serum tryptase 1-4 hours after symptom onset 1, 2
• The diagnostic threshold is an increase ≥20% above baseline PLUS an absolute increase ≥2 ng/mL 1, 2, 3
• Timing is critical—tryptase must be drawn within the 1-4 hour window after symptom onset for accurate interpretation 2

24-Hour Urine Mast Cell Mediator Metabolites

• Collect N-methylhistamine (NOT direct histamine, which is unreliable) 1
• Collect leukotriene E4, which peaks in 0-6 hour collections after episodes and can guide leukotriene antagonist therapy 1
• Collect 11β-prostaglandin F2α, which peaks in 0-3 hour collections and correlates with anaphylactic severity 1
• These urine markers are particularly useful when serum tryptase is difficult to obtain during acute episodes or remains negative 4

Genetic and Clonality Testing

Peripheral Blood Testing

• Test for KIT D816V mutation using highly sensitive allele-specific oligonucleotide quantitative PCR (ASO-qPCR) to identify clonal (primary) MCAS 1, 5
• Standard next-generation sequencing myeloid panels have only ~5% sensitivity for KIT D816V and should NOT be used for this purpose 5
• Obtain buccal swab for TPSAB1 α-tryptase copy number variation (CNV) testing to diagnose hereditary α-tryptasemia 1

When to Proceed to Bone Marrow Evaluation

• Bone marrow biopsy is indicated if baseline serum tryptase is persistently >20 ng/mL 1, 5, 2
• Also indicated if peripheral blood KIT D816V is positive, or if clinical features suggest systemic mastocytosis (adult-onset mastocytosis in the skin, abnormal blood counts, organomegaly) 1, 2
• Bone marrow analysis must include aspirate, core biopsy, immunohistochemistry (CD117, CD25, tryptase), flow cytometry for aberrant CD25 and CD2 expression, KIT D816V mutation analysis, and cytogenetics 1, 5, 2

Complete Blood Count and Additional Testing

• Obtain complete blood count with differential to evaluate for eosinophilia or other hematologic abnormalities 5
• If interstitial mast cell pattern with peripheral eosinophilia and negative KIT D816V, test for FIP1L1-PDGFRA fusion gene to exclude hypereosinophilic syndrome variants 5

Tests NOT Recommended (Common Pitfalls)

• Do NOT order plasma or urine histamine levels—these are unreliable; use N-methylhistamine instead 1
• Do NOT order heparin levels—heparin is not validated as a marker of mast cell activation 1
• Do NOT order chromogranin A—it resides in neuroendocrine cells, not mast cells, and is not a reliable marker 1

Practical Testing Algorithm

Initial Workup (All Suspected MCAS Patients)

• Baseline serum tryptase when asymptomatic 1, 5
• 24-hour urine collection for N-methylhistamine, leukotriene E4, and 11β-prostaglandin F2α 1, 5
• Complete blood count with differential 5
• Peripheral blood KIT D816V mutation by ASO-qPCR 1, 5
• Buccal swab for TPSAB1 α-tryptase CNV 1

During Symptomatic Episodes

• Acute serum tryptase 1-4 hours after symptom onset 1, 2
• Spot urine for mediator metabolites during symptoms if 24-hour collection is impractical 2

If Baseline Tryptase >20 ng/mL or Positive KIT D816V

• Proceed to bone marrow biopsy and aspirate with comprehensive analysis including immunohistochemistry, flow cytometry, and genetic testing 1, 5, 2

Classification After Testing

• If KIT D816V or TPSAB1 α-tryptase CNV is positive: Primary MCAS with somatic or germline mutation 4
• If both genetic tests are negative: MCAS without known mutation 4

Important Caveats

• ASO-qPCR detects KIT D816V in >80% of systemic mastocytosis patients when applied to bone marrow, but peripheral blood testing has lower sensitivity due to low allelic burden 5
• If peripheral blood KIT D816V is negative but clinical suspicion remains high (elevated baseline tryptase, recurrent symptoms), consider bone marrow testing for alternative codon 816 mutations or mutations in other KIT regions 5
• Some patients with clonal MCAS can progress to systemic mastocytosis, most likely indolent SM, making longitudinal monitoring important 4