What is the recommended workup for a patient with suspected Mast Cell Activation Syndrome (MCAS)?

MCAS Workup

The workup for suspected Mast Cell Activation Syndrome requires three parallel diagnostic tracks: documenting mast cell mediator elevation during symptomatic episodes, establishing baseline serum tryptase, and determining disease subtype through genetic and bone marrow evaluation when indicated. 1

Initial Laboratory Testing During Symptomatic Episodes

Serum Tryptase Measurement

• Obtain baseline serum tryptase when the patient is completely 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
• The diagnostic threshold is an increase ≥20% above baseline PLUS an absolute increase ≥2 ng/mL 1, 2
• This dual measurement approach (baseline vs. acute) is essential because tryptase is a preformed mediator and comparing levels optimizes diagnostic accuracy 1

24-Hour Urine Collection

• Collect N-methylhistamine (a histamine metabolite) rather than direct histamine measurement, which is not recommended due to poor reliability 1, 3
• Measure leukotriene E4 (LTE4) in 0-6 hour collections after episodes, as this can guide leukotriene antagonist therapy 1
• Measure 11β-prostaglandin F2α in 0-3 hour collections, which correlates with anaphylactic severity 1

Tests NOT to Order

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

Determining Disease Subtype: Clonality Assessment

Peripheral Blood Testing

• Perform KIT D816V mutation testing using highly sensitive allele-specific oligonucleotide quantitative PCR (ASO-qPCR) on peripheral blood to identify clonal (primary) MCAS 1, 4
• This highly sensitive assay can detect the mutation in peripheral blood first, avoiding unnecessary bone marrow procedures in many cases 4
• If peripheral blood is positive, this confirms clonal disease; if negative, bone marrow testing may still be needed 4

Hereditary Alpha-Tryptasemia Screening

• Use buccal swab for TPSAB1 α-tryptase copy number variation (CNV) to diagnose hereditary α-tryptasemia 1
• This genetic condition can predispose to more severe mast cell activation and should be identified 5

Bone Marrow Evaluation: When and What

Indications for Bone Marrow Biopsy

• Baseline serum tryptase persistently >20 ng/mL 1, 2
• Clinical features suggesting systemic mastocytosis (adult-onset mastocytosis in the skin, abnormal blood counts, organomegaly) 1
• Peripheral blood KIT D816V testing negative but high clinical suspicion remains 4

Components of Bone Marrow Analysis

• Bone marrow aspirate: Assess percentage of neoplastic mast cells, their morphology (spindle-shaped, well-differentiated, or immature), and percentage of abnormal mast cells out of total mast cells 4
• Core biopsy: Evaluate mast cell burden and whether mast cells form multifocal dense infiltrates (a major diagnostic criterion) or primarily interstitial pattern 4
• Immunohistochemistry: Perform staining for mast cell tryptase, CD117, and CD25 to optimize quantification; CD30 is optional but helpful when CD25 is negative 4
• Flow cytometry: Use CD117, CD25, and CD2 as standard markers; CD30 can also be considered 4
• Chromosome analysis: Obtain in all cases, especially when associated hematologic neoplasm is suspected 4
• Myeloid mutation panel: Perform on bone marrow (or peripheral blood if associated hematologic neoplasm or circulating mast cells present), but note that NGS assays have low sensitivity (~5%) for KIT D816V 4
• Reticulin and collagen staining: Assess grade of bone marrow fibrosis (MF-0 to MF-3), common in advanced disease 4

Special Bone Marrow Testing Considerations

• If primarily interstitial pattern of mast cells with peripheral blood eosinophilia and negative KIT D816V, test for FIP1L1-PDGFRA fusion gene 4
• KIT mutational analysis on bone marrow can be performed on formalin-fixed paraffin-embedded tissue if not decalcified or decalcified in EDTA only; other fixatives yield unsatisfactory results 4
• In cases of suboptimal bone marrow aspirate (dry tap), test peripheral blood as alternative for KIT D816V detection 4

Diagnostic Algorithm Summary

Step 1: Clinical Recognition

• Identify recurrent, episodic symptoms affecting at least two organ systems concurrently (cardiovascular, respiratory, dermatologic, gastrointestinal) 2, 6

Step 2: Mediator Documentation

• Obtain baseline tryptase when asymptomatic 1
• During symptomatic episode: acute tryptase (1-4 hours post-onset) and/or 24-hour urine for N-methylhistamine, LTE4, and 11β-PGF2α 1
• Document elevation meeting diagnostic thresholds on at least two separate occasions 2

Step 3: Subtype Classification

• Peripheral blood KIT D816V mutation testing 1
• Buccal swab for TPSAB1 α-tryptase CNV 1
• Bone marrow evaluation if tryptase >20 ng/mL persistently or clinical features suggest systemic mastocytosis 1, 4

Step 4: Exclude Secondary Causes

• Rule out IgE-mediated allergies, drug reactions, and infections before confirming MCAS diagnosis 1

Step 5: Confirm Treatment Response

• Document improvement with mast cell-targeted therapies (H1/H2 antihistamines, mast cell stabilizers, leukotriene modifiers) as this is a required diagnostic criterion 2, 1

Critical Pitfalls to Avoid

• MCAS is substantially overdiagnosed - do not diagnose based solely on nonspecific symptoms, single organ system involvement, or symptoms without documented mediator elevation 2
• Do not use standard NGS myeloid panels for KIT D816V detection - sensitivity is only ~5%; use highly sensitive ASO-qPCR instead 4
• Do not skip baseline tryptase measurement - acute tryptase alone without baseline comparison is insufficient for diagnosis 1
• In <5-10% of patients, no KIT D816V mutation is detected despite true disease, either due to very low mast cell burden causing false-negative results or presence of other rare KIT mutations 4
• Multiple conditions can coexist (hereditary alpha tryptasemia, clonal disease, IgE-mediated allergies) - each requires identification for optimal management 5