Mast Cell Disorders

Medical Experts

Author

George

Tracy I. George, MD

Professor of Pathology (Clinical), University of Utah

President and Chief Scientific Officer, and Medical Director, Hematopathology, ARUP Laboratories

Author

March

Jordon K. March, MD

Former Anatomic and Clinical Pathology Resident, University of Utah School of Medicine and ARUP Laboratories

Former Associate Medical Director, ARUP Consult

Mast cell disorders (MCDs), including mast cell activation syndrome (MCAS) and mastocytosis, are a diverse group of conditions characterized by inappropriate mast cell activation and/or the proliferation and accumulation of abnormal mast cells throughout the body.^, These conditions range in severity from benign, nonclonal disorders to malignant clonal diseases that rapidly progress and may involve extracutaneous organs (eg, systemic mastocytosis [SM]) or the skin (eg, cutaneous mastocytosis [CM]). Symptoms result from the release of mast cell mediators (eg, tryptase and histamine) and/or end organ damage. The diagnostic approach depends on the type of MCD but often involves laboratory testing such as serum tryptase measurement and KIT gene mutational analysis on bone marrow and/or peripheral blood.

Quick Answers for Clinicians

What are the advantages of testing methods such as digital droplet polymerase chain reaction for the diagnosis of mast cell disorders?

Ultrasensitive testing methods such as digital droplet polymerase chain reaction (ddPCR) are the preferred first-line methods for KIT mutational analysis to diagnose mast cell disorders (MCDs), given that these methods are sensitive, highly specific, and can be performed on peripheral blood. KIT D816V mutations are detected in the blood of the majority of patients with systemic mastocytosis (SM) when ultrasensitive techniques are used, which eliminates the need for invasive bone marrow biopsy in these individuals. However, a small percentage of individuals with SM will be negative for KIT D816V in peripheral blood even when an ultrasensitive technique is used.

What is the appropriate timing for tryptase testing in mast cell disorders?

When evaluating an individual for nonclonal mast cell activation syndrome (NC-MCAS), an increase in the serum level of tryptase above baseline and within a narrow window of time (generally 30 minutes to 2 hours) after a symptomatic episode is the preferred method to provide evidence of mast cell involvement. Alternatively, if a clonal process is suspected, serum tryptase can be tested at any time. KIT mutational testing can also be performed at any time.

How should the serum tryptase level be adjusted in the presence of hereditary alpha-tryptasemia to apply the diagnostic criteria for systemic mastocytosis?

There are multiple options to adjust serum tryptase in hereditary alpha-tryptasemia, including division of the total serum tryptase level by a number equal to one plus the number of copies of TPSAB1 present, in accordance with World Health Organization recommendations. Alternatively, online calculators are available, which may allow for genotype-specific adjustments.^, Refer to the National Comprehensive Cancer Network (NCCN) Systemic Mastocytosis guidelines for more information.

When should testing for mast cell disorders be performed in children?

Evaluation is appropriate in children with^,:

Nonpolymorphic urticaria pigmentosa (particularly at ≤1 year of age) and Darier’s sign
Elevated serum tryptase concentrations
Organomegaly
Other signs and symptoms (eg, unexplained CBC abnormalities)

Testing should include serum tryptase and peripheral blood testing for the KIT D816V mutation. Because systemic mastocytosis (SM) is rare in children, invasive bone marrow testing should only be performed if there is high suspicion for SM or another hematologic malignancy or if cutaneous manifestations persist beyond 18 years of age.

Indications for Testing

In adults, testing is appropriate in individuals with^,^,^,:

Recurrent anaphylaxis (particularly with a Spanish Network on Mastocytosis [REMA] score ≥2)
Elevated serum tryptase concentrations
Urticaria pigmentosa (mastocytosis of the skin) confirmed by skin biopsy
Other signs and symptoms of mast cell activation (eg, diarrhea, fainting, flushing, pruritis, rash, wheezing)
A previous mastocytosis diagnosis

Diagnostic Criteria and Classification

Diagnostic Criteria for Systemic Mastocytosis

The major and minor diagnostic criteria for SM are based on the results of laboratory testing.

Major SM criterion^,:

Multifocal dense infiltrates of mast cells (≥15 mast cells in aggregates) are detected in bone marrow biopsies and/or in sections of other extracutaneous organ(s).

Minor SM criteria^,:

At least 25% of all mast cells are atypical cells (type I or type II) on bone marrow smears or are spindle shaped in mast cell infiltrates detected on histologic sections of visceral organ(s).
Any activating KIT point mutation (eg, KIT D816V) is detected in bone marrow or another extracutaneous organ.
Mast cells in bone marrow, blood, or other extracutaneous organs express CD2, CD25, and/or CD30.
Serum total tryptase level persistently exceeds 20 ng/mL (unless there is an associated clonal myeloid disorder; adjusted if there is hereditary alpha-tryptasemia).

Diagnosis and Classification Based on Criteria for Systemic Mastocytosis

The diagnosis and classification of MCDs is based on the above SM criteria and other findings:

Diagnosis and Classification of MCDs
Diagnosis	Criteria or Findings
SM^a	WHO: Major SM criterion and 1 minor SM criterion OR ≥3 minor SM criteria ICC: Major SM criterion OR ≥3 minor SM criteria
CM^a	Skin lesions typical of urticaria pigmentosa AND Criteria for SM not met
Mast cell sarcoma	Solitary malignant mast cell neoplasm
Monoclonal (also referred to as primary) MCAS	KIT D816V and/or CD25 mast cells on immunophenotyping AND <3 minor SM criteria
Hereditary alpha-tryptasemia^b	TPSAB1 copy number variants High serum tryptase level Potentially, other signs and symptoms of mast cell activation
^aSM and CM may be subclassified according to additional findings. WHO and ICC subclassifications differ; refer to the NCCN guidelines for more information. ^bMay occur on its own or in conjunction with mastocytosis. ICC, International Consensus Classification; NCCN, National Comprehensive Cancer Network; WHO, World Health Organization Sources: NCCN, 2024; Khoury, 2022; Arber, 2022

SM and CM may be subclassified according to additional findings. WHO and ICC subclassifications for SM differ^,; refer to the NCCN guidelines for more information.

Laboratory Testing

Initial Workup

Recommended laboratory testing in the initial workup of suspected MCDs is similar to testing for many other conditions and includes a CBC with differential, metabolic panel, peripheral smear, and lactate dehydrogenase, liver function, and uric acid tests, as well as testing for mast cell mediators.

Testing for Mast Cell Mediators

The manifestations of mast cell activation are broad, and patients may present for evaluation due to a variety of symptoms or abnormal test results (eg, abnormal CBC or elevated serum tryptase). One suggested approach to avoid both missed diagnoses and unnecessary testing is to tailor the evaluation of potential MCDs to the clinical presentation.^, Factors that may guide the next steps in testing include the presence of skin lesions and the REMA score (used to predict mastocytosis using factors from the clinical examination and tryptase concentrations).^, Recently, it has been demonstrated that adjusting the basal serum tryptase level according to the tryptase genotype improves the sensitivity and specificity of mastocytosis evaluation in individuals with indolent SM and that determining personalized, genotype-specific reference basal serum tryptase ranges may be useful; therefore, genetic testing may be reasonable to perform with tryptase testing at presentation in adults without skin lesions. Refer to the Mast Cell Disorders Testing Algorithm for a suggested stepwise testing strategy.

Serum Tryptase

Serum tryptase is an important initial test in the evaluation of patients with a suspected MCD, although there are limitations to its sensitivity and specificity for the evaluation of mastocytosis. Mast cells in the tissue produce and constitutively release the alpha form of tryptase, which can be detected by a commercial fluoroimmune enzyme assay.

When evaluating patients for nonclonal MCAS, a baseline level should be obtained, and tryptase should be measured again during or shortly after a symptomatic episode.^, Ideally, a sample should be collected 30-120 minutes after onset, although testing may be useful up to 6 hours after onset. If not collected before onset, a baseline sample may be obtained 24 or more hours after all symptoms have ceased. A 20% plus 2 ng/mL increase in serum tryptase from the baseline level constitutes mast cell activation.^,^,^,

If a clonal process is suspected (eg, SM), serum tryptase can be tested at any time. In individuals with SM who have experienced anaphylaxis or mast cell activation, serum tryptase should be measured within 2 hours of onset and again after recovery for comparison.

The basal serum tryptase level is ≥8 ng/mL (high normal to high) in hereditary alpha-tryptasemia. (6- NCCN) If hereditary alpha-tryptasemia has been diagnosed, basal serum tryptase levels should be adjusted to properly apply the diagnostic criteria for SM. There are multiple options to adjust serum tryptase in hereditary alpha-tryptasemia.

Serum tryptase can be elevated in other hematologic myeloid malignancies, including myelodysplastic syndromes and acute leukemia, as well as chronic kidney disease; therefore, follow-up of elevated tryptase should include bone marrow aspiration and biopsy, after renal disease is excluded. Refer to the Bone Marrow Examination section.

Interpretation of Serum Tryptase Results
Tryptase Result (ng/mL)	Interpretation
8	Minimum baseline concentration in individuals with hereditary alpha-tryptasemia 95% threshold for individuals without hereditary alpha-tryptasemia or MCD (<5% have BST ≥8 ng/mL)
<11.4	Considered “normal” by most laboratories
>11.4	Abnormally elevated when considering diagnosis of clonal MCD in the absence of hereditary alpha-tryptasemia
>20.0	Minor criterion for diagnosis of SM (unless another myeloid neoplasm is present) If elevated on 2 separate occasions, evaluation for SM should be pursued Should be adjusted if hereditary alpha-tryptasemia is present
≥125	Variable in the IPSM score for advanced SM, global prognostic score model for progression-free survival, global prognostic score model for overall survival
>200	B-finding indicating higher disease burden for individuals with SM
BST, basal serum tryptase; IPSM, International Prognostic Scoring System in Mastocytosis Sources: Hamilton, 2018; NCCN, 2024; Chovanec, 2023

Other Mast Cell Mediators

Although serum tryptase is the only mast cell mediator included in consensus criteria, other laboratory tests that can be considered include 24-hour urine tests or random urine tests for histamine, leukotriene C4, N-methylhistamine, 2,3-Dinor-11beta-prostaglandin F2 alpha, and prostaglandin D2.^,^, These markers are most commonly used for the evaluation of mast cell activation and may be useful if serum tryptase testing is unavailable, but their sensitivity and/or specificity are limited; therefore, they should not be used for MCD evaluation.^, They may also be helpful in targeting therapy for symptoms of mast cell activation, which may be directed at the elevated mediator.^,

Diagnosis of Mastocytosis

The NCCN recommends that the evaluation for suspected SM include a biopsy (bone marrow or involved organ), immunophenotyping of mast cells (flow cytometry or immunohistochemistry [IHC]), and KIT gene testing.

Bone Marrow Examination

Bone marrow biopsy remains the gold standard for establishing the diagnosis of mastocytosis. When bone marrow is examined for MCD, good practice is to examine a blood smear, a bone marrow aspirate smear, and a bone marrow biopsy. The following ancillary tests are highly useful in this situation^,:

Chromosomal analysis (karyotype) on bone marrow aspirate
Fluorescence in situ hybridization (FISH) for eosinophilia (when eosinophilia is present) on blood or aspirate
Ultrasensitive KIT D816V testing on aspirate
Myeloid mutation panel (eg, containing SRSF2, ASXL1, RUNX1) on bone marrow aspirate
IHC on bone marrow biopsy for tryptase and CD117
Immunophenotyping of mast cells for CD2, CD25, and CD30 via flow cytometry and/or IHC

Tests for other markers are performed based on pathology findings.

KIT Mutational Analysis

All individuals suspected of having SM should undergo KIT mutational analysis. KIT testing is typically performed on bone marrow or peripheral blood. Traditionally, KIT mutational testing was performed on only bone marrow because bone marrow lesional tissue has a higher concentration of mast cells. However, high-sensitivity molecular testing (eg, digital droplet polymerase chain reaction [ddPCR]) can be used to detect the mutation in the peripheral blood of most adult patients with typical SM.^,

In adult patients, an activating KIT p.D816V mutation is found in the majority of cases of SM. Other KIT mutations in SM include the deletion of codon 419 on exon 8, A502_Y503dup in exon 9, NM_000222:KIT p.F522C, and NM_000222:KIT p.V560G. Use of massively parallel sequencing (also referred to as next generation sequencing, or NGS) is recommended to test for other KIT mutations, particularly if KIT D816V is negative.

If a KIT D816V mutation is detected in peripheral blood, a bone marrow biopsy should be performed to evaluate for SM, including the subtype of the disease. Evaluation for the presence of an associated hematologic neoplasm such as acute myeloid leukemia, a myelodysplastic syndrome, or a myeloproliferative neoplasm should be performed. Bone marrow biopsy should also be performed in cases with a high suspicion of SM, even when no KIT mutation is detectable in the peripheral blood.

Additional Genetic Testing

If KIT D816V testing is negative and eosinophilia is present, testing for the FIP1L1::PDGFRA fusion gene is recommended.

Tryptase copy number determination is helpful in establishing a diagnosis of hereditary alpha-tryptasemia, which may occur on its own or in conjunction with mastocytosis. Hereditary alpha-tryptasemia is associated with additional copies of TPSAB1.^, Refer to the Testing for Mast Cell Mediators section for more information about the utility of tryptase genotyping in testing for MCDs.

Additional somatic mutations in ASXL1, JAK2, RUNX1, SRSF2, TET2, or other genes may be present in individuals with KIT D816V and may be associated with another hematologic malignancy and/or observed in individuals with advanced SM.

Human leukocyte antigen (HLA) testing may be performed if allogeneic hematopoietic stem cell transplantation is being considered for treatment. For more information on HLA transplantation testing, refer to the ARUP Consult HLA Testing topic.

Monitoring

Laboratory monitoring is recommended every 6-12 months for SM and yearly for indolent SM. Laboratory tests that may be useful for assessment include serum tryptase measurements of mast cell burden, CBCs, and chemistry panels.

Serial measurements of the KIT D816V allele burden by ultrasensitive quantitative PCR techniques appear useful to monitor residual disease in aggressive subtypes during or after cytoreductive therapy or allogeneic stem cell transplantation. The frequency of measurement of the allele burden should be adapted to the individual patient situation. Thus, in patients who have indolent SM with low mast cell burden and stable clinical course, the KIT D816V allele burden should be measured at diagnosis, but measurement should not necessarily be repeated unless signs of disease progression occur. In patients with more advanced forms of disease who are enrolled in clinical trials with cytoreductive therapies, the KIT D816V allele burden should be measured repeatedly before and during therapy.

When symptoms worsen, a bone marrow aspiration and biopsy should be considered.