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MCAS, mastocytosis and tryptase
Understand mast-cell assessment without overinterpreting results
In this sheet
The essentials in 30 seconds
Mast cells are a normal part of the immune system
When activated, they can release histamine, tryptase and other mediators.
Symptoms alone are not enough to diagnose MCAS
The consensus diagnosis requires compatible systemic episodes, biological evidence of activation and a response to treatments targeting mast-cell mediators.
Acute tryptase is compared with your baseline tryptase
A value within the laboratory’s ‘normal’ range may still have risen significantly above your usual value.
Elevated baseline tryptase does not automatically mean mastocytosis
Hereditary alpha-tryptasemia and other situations can also raise baseline tryptase.
Mastocytosis is not diagnosed from tryptase alone
Diagnosis is based on a combination of clinical, laboratory, molecular and sometimes bone-marrow criteria.
During anaphylaxis, treatment comes before testing
Never delay adrenaline, the emergency call or urgent care in order to have a blood test.
What is a mast cell?
Mast cells are immune cells found in many tissues, including the skin, airways and digestive tract. They contain mediators that can be released when the cells are activated.
- histamine
- tryptase
- leukotrienes
- prostaglandins
- other inflammatory mediators
Varied but non-specific symptoms
Mast-cell activation may cause flushing, itching, urticaria, digestive symptoms, breathing difficulty, feeling faint or a fall in blood pressure. These symptoms can also have many other causes.
Mast-cell activation, MCAS and mastocytosis: what is the difference?
- Concept
- Mast-cell activation
- Definition
- A biological phenomenon that may occur, for example, during an allergic reaction or anaphylaxis. A single episode of activation does not mean that you have MCAS.
- Concept
- MCAS
- Definition
- Mast cell activation syndrome meeting precise diagnostic criteria. A list of non-specific symptoms is not enough.
- Concept
- Mastocytosis
- Definition
- A clonal disease characterised by an abnormal accumulation or proliferation of mast cells in one or more tissues. It may be cutaneous or systemic.
| Concept | Definition |
|---|---|
| Mast-cell activation | A biological phenomenon that may occur, for example, during an allergic reaction or anaphylaxis. A single episode of activation does not mean that you have MCAS. |
| MCAS | Mast cell activation syndrome meeting precise diagnostic criteria. A list of non-specific symptoms is not enough. |
| Mastocytosis | A clonal disease characterised by an abnormal accumulation or proliferation of mast cells in one or more tissues. It may be cutaneous or systemic. |
MCAS: what are the criteria?
The most widely used consensus criteria require all three of the following dimensions:
- Recurrent episodes compatible with systemic mast-cell activation, usually involving several organ systems.
- Biological evidence of increased mast-cell mediators during the episode, with tryptase as the reference biomarker.
- Improvement with treatment targeting mast-cell activation or mediators.
Why and when is tryptase measured?
- Sample
- Acute tryptase
- Purpose
- Taken during or shortly after a suspected episode. It looks for a temporary rise associated with mast-cell activation.
- Sample
- Baseline tryptase
- Purpose
- Taken after you have returned to your usual state. It provides a personal reference value and may also guide some investigations.
| Sample | Purpose |
|---|---|
| Acute tryptase | Taken during or shortly after a suspected episode. It looks for a temporary rise associated with mast-cell activation. |
| Baseline tryptase | Taken after you have returned to your usual state. It provides a personal reference value and may also guide some investigations. |
During the reaction
When this can be done without delaying treatment, acute tryptase should ideally be taken as soon as possible, preferably within 2 hours of symptom onset. A window of up to about 4 hours may still be useful depending on the context.
For the baseline value
Baseline tryptase is measured away from the episode, after symptoms have fully resolved, usually at least 24 hours later.
Safety first
Anaphylaxis is a clinical diagnosis. Tryptase sampling must never delay adrenaline, calling emergency services or other urgent care.
Understanding the ‘20% + 2’ rule
To look for a significant rise in tryptase during an episode, the standard consensus formula compares the acute value with your baseline value:
This calculation does not establish the diagnosis on its own
SOS Allergo can explain the formula, but an MCAS diagnosis requires all the criteria and medical interpretation. Elevated baseline tryptase, particularly with hereditary alpha-tryptasemia or mastocytosis, requires specialist interpretation.
Does ‘normal’ tryptase rule out anaphylaxis?
No. Anaphylaxis is primarily a clinical diagnosis. Tryptase may remain within the laboratory’s usual range, particularly in some reactions. Comparison with your personal baseline is often more informative than a simple ‘normal / abnormal’ result.
Why is my baseline tryptase elevated?
Persistently elevated baseline tryptase can have several causes. It must be interpreted alongside your history and other investigations.
- hereditary alpha-tryptasemia (HαT)
- mastocytosis
- certain haematological diseases
- kidney failure and a few other clinical situations
What is hereditary alpha-tryptasemia?
Some people have extra copies of the TPSAB1 gene encoding alpha-tryptase. This often causes a higher baseline tryptase and is known as hereditary alpha-tryptasemia (HαT).
Genetic testing may be considered in certain situations, particularly when baseline tryptase is elevated or during some anaphylaxis or mastocytosis assessments.
What is mastocytosis?
Mastocytosis is an abnormal clonal proliferation of mast cells. It may mainly involve the skin or affect other organs. In adults, many forms are indolent, but assessment depends on the type of disease and symptoms.
How is mastocytosis investigated?
Depending on your situation, the specialist may suggest some or all of the following investigations:
- skin examination and search for suggestive lesions
- baseline tryptase
- full blood count and other laboratory tests
- sensitive testing for a KIT mutation, particularly D816V
- bone assessment where relevant
- bone-marrow examination in selected situations
- other investigations guided by symptoms and context
Why is my allergist investigating further?
Some situations may prompt a more active search for clonal mast-cell disease, for example:
- severe or repeated anaphylaxis without a clear cause
- severe reaction to Hymenoptera venom
- persistently elevated baseline tryptase
- skin lesions suggestive of mastocytosis
- unexplained vertebral fracture or osteoporosis
- haematological abnormalities or other particular signs
Further information: different types of MCAS
- Category
- Secondary MCAS
- Key information
- Mast-cell activation is explained by an identifiable cause, such as an allergy.
- Category
- Primary / clonal MCAS
- Key information
- A clonal mast-cell population is identified.
- Category
- Idiopathic MCAS
- Key information
- MCAS criteria are met, but no cause is found after an appropriate assessment.
| Category | Key information |
|---|---|
| Secondary MCAS | Mast-cell activation is explained by an identifiable cause, such as an allergy. |
| Primary / clonal MCAS | A clonal mast-cell population is identified. |
| Idiopathic MCAS | MCAS criteria are met, but no cause is found after an appropriate assessment. |
Common misconceptions
- Misconception
- ‘Elevated tryptase means mastocytosis.’
- SOS Allergo guidance
- No. Several situations can raise baseline tryptase.
- Misconception
- ‘My tryptase is normal, so it was not anaphylaxis.’
- SOS Allergo guidance
- No. Anaphylaxis is a clinical diagnosis and tryptase may remain within the usual range.
- Misconception
- ‘I have many symptoms, so I must have MCAS.’
- SOS Allergo guidance
- No. MCAS has precise diagnostic criteria.
- Misconception
- ‘MCAS and mastocytosis are the same disease.’
- SOS Allergo guidance
- No. Mast-cell activation and a clonal mast-cell disease are different concepts.
- Misconception
- ‘HαT is a form of mastocytosis.’
- SOS Allergo guidance
- No. It is a genetic trait affecting baseline tryptase.
- Misconception
- ‘Tryptase must be measured before treating anaphylaxis.’
- SOS Allergo guidance
- Absolutely not. Urgent treatment comes before sampling.
| Misconception | SOS Allergo guidance |
|---|---|
| ‘Elevated tryptase means mastocytosis.’ | No. Several situations can raise baseline tryptase. |
| ‘My tryptase is normal, so it was not anaphylaxis.’ | No. Anaphylaxis is a clinical diagnosis and tryptase may remain within the usual range. |
| ‘I have many symptoms, so I must have MCAS.’ | No. MCAS has precise diagnostic criteria. |
| ‘MCAS and mastocytosis are the same disease.’ | No. Mast-cell activation and a clonal mast-cell disease are different concepts. |
| ‘HαT is a form of mastocytosis.’ | No. It is a genetic trait affecting baseline tryptase. |
| ‘Tryptase must be measured before treating anaphylaxis.’ | Absolutely not. Urgent treatment comes before sampling. |
Preparing for specialist assessment
If possible, gather a timeline of episodes, tryptase results with their dates and times, medical reports and prescriptions.
Prepare for your MCAS / mastocytosis appointmentKey points
- Mast-cell activation, MCAS and mastocytosis are not synonymous.
- MCAS is not diagnosed from non-specific symptoms alone.
- Acute tryptase must be compared with a personal baseline value.
- The 20% + 2 rule is a biological criterion for a significant rise, not a standalone diagnosis.
- Elevated baseline tryptase may be related to HαT or mastocytosis, among other causes.
- Tryptase >20 ng/mL is a minor criterion for systemic mastocytosis, not sufficient proof.
- During anaphylaxis, adrenaline and urgent care must never be delayed for testing.
You may also want to read
Main references
- Bulai Livideanu C, Barete S, Damaj G, et al. French guidelines for the management of nonadvanced mastocytosis in adults. Orphanet J Rare Dis. 2025;20:499. doi:10.1186/s13023-025-03764-7.
- Gülen T. Using the Right Criteria for MCAS. Curr Allergy Asthma Rep. 2024;24(2):39-51. doi:10.1007/s11882-024-01126-0.
- Valent P, Akin C, Hartmann K, et al. Updated Diagnostic Criteria and Classification of Mast Cell Disorders: A Consensus Proposal. HemaSphere. 2021;5:e646.
- Golden DBK, Wang J, Waserman S, et al. Anaphylaxis: A 2023 practice parameter update. Ann Allergy Asthma Immunol. 2024.
- Rama TA, Gülen T. Hereditary Alpha-Tryptasemia and Mastocytosis: What We Know and What We Need To Learn. Curr Allergy Asthma Rep. 2026;26:19. doi:10.1007/s11882-026-01262-9.
- CEREMAST / AP-HP. 100 questions / réponses sur la mastocytose. Ressource patient française.
Scientific review: September 2026 · sosallergo.fr
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