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allergy.mast-cell-activation-syndrome.core.v1PRODUCTION
allergy.mast-cell-activation-syndrome.core.v1

Mast cell activation syndrome (MCAS)

allergychronicsyndromeadultpediatric
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12/12 authored

Canonical 12-phase frame with authored status for this dossier.

Current phase

Frame

Detailed

Frame MCAS as a recurrent multi-organ syndrome requiring ALL THREE consensus criteria (multi-organ symptoms + objective mediator-release + response to MC-directed therapy) — NOT a diagnosis assignable on symptoms alone. Distinguish primary (clonal, KIT D816V) vs secondary (IgE-mediated) vs idiopathic; recognise HαT as a tryptase modifier, not a stand-alone MCAS diagnosis.

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Three-criteria diagnostic frame set; ddx of HαT vs clonal MC disease noted

Patient inputs (10)

Common MC triggers (heat, exercise, alcohol, stress, certain meds — NSAIDs, opioids, vancomycin, neuromuscular blockers — Hymenoptera, foods) — drives avoidance counselling (Valent 2020 PMID 33261124)

Diagnosis criterion 1 requires recurrent typical symptoms across >=2 organ systems (skin/GI/respiratory/CV/neurologic) (Valent 2020 PMID 33261124)

Diagnosis criterion 2 — objective mediator-release evidence; "20+2 rule": rise >=20% over individual basal + 2 ng/mL within 4h of event (Valent 2020 PMID 33261124)

Required denominator for the 20+2 rule; elevated basal levels prompt HαT or clonal-MC workup (Chovanec 2023 PMID 36170795)

Diagnosis criterion 3 — objective improvement on H1+H2+cromolyn+LTRA trial supports MCAS (Valent 2020 PMID 33261124)

Pediatric mastocytosis differs (cutaneous mastocytosis often regresses); HαT prevalence affects family screening; agent age cutoffs differ

Patients with anaphylactic-grade events MUST carry epinephrine auto-injectors; technique re-taught every visit

KIT D816V allele assay (peripheral blood digital PCR is sensitive; bone marrow biopsy is gold standard) — separates primary clonal MCAS / systemic mastocytosis from idiopathic/secondary MCAS (Valent 2020 PMID 33261124)

TPSAB1 copy-number genotyping resolves elevated basal tryptase attributable to hereditary alpha-tryptasemia vs clonal mast cell disease (Lyons 2016 PMID 27749843; Chovanec 2023 PMID 36170795)

Alternative mediator-release evidence when tryptase capture is impractical or non-diagnostic (Valent 2020 PMID 33261124)

* = hard-required. Engine cannot meaningfully run until these are filled.

Severity triggers (5)

5 need judgement
  • informationallife_threateninganaphylactic_grade_event_in_mcas
    Anaphylactic-grade event (two-organ system / CV / airway) in known or suspected MCAS
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalmoderateclonal_mcas_or_systemic_mastocytosis_workup
    Basal tryptase >20 ng/mL persistent (or HαT-corrected upper limit exceeded), KIT D816V positive, cytopenias, hepatosplenomegaly, lymphadenopathy
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalmoderatepre_procedure_premedication_for_mcas
    Planned contrast study, surgery, dental procedure, or anaesthesia in patient with MCAS / clonal MC disease
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalmoderateidiopathic_recurrent_anaphylaxis
    Recurrent anaphylaxis with negative allergic and clonal workup
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalmildhereditary_alpha_tryptasemia_identified
    TPSAB1 copy-number duplication/triplication confirmed; basal tryptase elevated proportional to TPSAB1 dose
    Trigger could not be auto-evaluated — needs clinician judgement.

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Recommended regimen

MCAS — H1+H2 antihistamine foundation + cromolyn + LTRA + ketotifen + omalizumab; clonal disease via heme-onc
axis: mcas_mediator_directed_stepwise_ladderstep 1 - Step 1 — Trigger avoidance + epinephrine auto-injector + written action plan (every patient, lifelong)
Selected step "Step 1 — Trigger avoidance + epinephrine auto-injector + written action plan (every patient, lifelong)" — All MCAS patients, all severities — the non-negotiable substrate
  • trigger_avoidance_inventory
    first line
    avoidance
    Valent 2020 (PMID 33261124) — heat, exercise, alcohol, NSAIDs, opioids, vancomycin, NMBA, Hymenoptera, foods; personalised inventory drives prevention.
  • epinephrine
    rescue
    alpha_beta_adrenergic_agonist_auto_injector
    0.15 mg IM (peds <25 kg) / 0.3 mg IM (>=25 kg) • IM lateral thigh • PRN for systemic reaction; may repeat 5-15 min (max: Per device; two devices carried)
    triggers: systemic_reaction_in_progress
    Valent 2020 (PMID 33261124) — anaphylaxis-grade MCAS events require IM epinephrine first-line; two auto-injectors carried; technique re-taught.
    rxcui 3992
  • written_mcas_action_plan
    first line
    patient_education
    Patient education on early-symptom recognition + step-up therapy + when to use auto-injector + when to seek care.

outpatient playbook — drug actions (6)

  1. 1. epinephrine auto-injector x 2 + action plan
    rxcui 3992
    0.15 / 0.3 mg IM (weight-based) • IM • PRN systemic reaction
    trigger: Any patient with anaphylactic-grade history (Valent 2020 PMID 33261124)
    Rescue path for systemic reactions; two devices
  2. 2. cetirizine + famotidine foundation
    rxcui 20610
    10 mg PO daily-QID + famotidine 20-40 mg BID • PO • daily-BID/QID
    trigger: Confirmed MCAS (Valent 2020 PMID 33261124)
    H1+H2 antihistamine baseline; often uptitrated above label in MCAS
  3. 3. cromolyn for GI-dominant
    rxcui 42612
    200 mg PO QID • PO • QID
    trigger: GI dominant phenotype or refractory step 2 (Valent 2020 PMID 33261124)
    Local MC stabilisation in gut; minimal systemic absorption
  4. 4. montelukast or zileuton for LT-dominant phenotype
    rxcui 88249
    10 mg PO daily • PO • daily evening
    trigger: LT-mediated dominant symptoms (Valent 2020 PMID 33261124)
    Counsel montelukast neuropsychiatric warning; zileuton needs LFT monitoring
  5. 5. omalizumab for refractory / idiopathic anaphylaxis
    rxcui 302379
    300-600 mg SC q2-4wk • SC • q2-4wk
    trigger: Refractory MCAS or idiopathic anaphylaxis (Valent 2020 PMID 33261124; Giannetti 2020 PMID 32276688)
    Off-label; durable benefit reported
  6. 6. midostaurin for advanced SM via heme-onc
    rxcui 1919083
    100 mg PO BID • PO • BID
    trigger: Advanced systemic mastocytosis (heme-onc co-management)
    FDA approved for aggressive SM, SM-AHN, mast cell leukaemia

Auto-drafted A&P note

outpatient

Subjective

- Possible entry pathways: Recurrent episodic typical mast-cell symptoms involving >=2 organ systems (Valent 2020 PMID 33261124); Recurrent unexplained anaphylaxis without an identifiable IgE trigger — idiopathic anaphylaxis as a form of MCAS (Giannetti 2020 PMID 32276688); Elevated basal serum tryptase (>11.4 ng/mL) — workup for clonal mast-cell disease vs HαT (Lyons 2016 PMID 27749843; Chovanec 2023 PMID 36170795).

Objective

- No vitals, labs, or imaging entered for this encounter.

Assessment

**Mast cell activation syndrome (MCAS)** (allergy.mast-cell-activation-syndrome.core.v1).
Phenotype framing: Terminal differential: MCAS vs systemic mastocytosis (WHO criteria — major: dense MC infiltrates on bone marrow + KIT D816V; minor: atypical MC morphology, tryptase >20, aberrant CD25/CD2/CD30 on MC) vs hereditary alpha-tryptasemia alone vs IgE-mediated allergy with anaphylaxis vs carcinoid syndrome (5-HIAA, neuroendocrine tumour) vs paroxysmal autonomic dysfunction vs hereditary angioedema (C1-INH deficiency, no urticaria, no MC mediator rise) vs phaeochromocytoma vs panic / functional disorder.
Scope: Frame MCAS as a recurrent multi-organ syndrome requiring ALL THREE consensus criteria (multi-organ symptoms + objective mediator-release + response to MC-directed therapy) — NOT a diagnosis assignable on symptoms alone. Distinguish primary (clonal, KIT D816V) vs secondary (IgE-mediated) vs idiopathic; recognise HαT as a tryptase modifier, not a stand-alone MCAS diagnosis.

No severity triggers fired against current inputs.

Plan

Regimen axis: **MCAS — H1+H2 antihistamine foundation + cromolyn + LTRA + ketotifen + omalizumab; clonal disease via heme-onc** — step "Step 1 — Trigger avoidance + epinephrine auto-injector + written action plan (every patient, lifelong)".
1. trigger_avoidance_inventory (avoidance, first line) — Valent 2020 (PMID 33261124) — heat, exercise, alcohol, NSAIDs, opioids, vancomycin, NMBA, Hymenoptera, foods; personalised inventory drives prevention.
2. epinephrine 0.15 mg IM (peds <25 kg) / 0.3 mg IM (>=25 kg) IM lateral thigh PRN for systemic reaction; may repeat 5-15 min (alpha_beta_adrenergic_agonist_auto_injector, rescue) — Valent 2020 (PMID 33261124) — anaphylaxis-grade MCAS events require IM epinephrine first-line; two auto-injectors carried; technique re-taught.
3. written_mcas_action_plan (patient_education, first line) — Patient education on early-symptom recognition + step-up therapy + when to use auto-injector + when to seek care.

Setting playbook (outpatient) — Apply the three-criteria diagnostic frame (multi-organ symptoms + objective mediator-release evidence + response to MC-directed therapy), classify primary vs secondary vs idiopathic, recognise HαT, and titrate the stepwise ladder while keeping epinephrine and an action plan in place (Valent 2020 PMID 33261124)
4. epinephrine auto-injector x 2 + action plan 0.15 / 0.3 mg IM (weight-based) IM PRN systemic reaction — Any patient with anaphylactic-grade history (Valent 2020 PMID 33261124) (Rescue path for systemic reactions; two devices)
5. cetirizine + famotidine foundation 10 mg PO daily-QID + famotidine 20-40 mg BID PO daily-BID/QID — Confirmed MCAS (Valent 2020 PMID 33261124) (H1+H2 antihistamine baseline; often uptitrated above label in MCAS)
6. cromolyn for GI-dominant 200 mg PO QID PO QID — GI dominant phenotype or refractory step 2 (Valent 2020 PMID 33261124) (Local MC stabilisation in gut; minimal systemic absorption)
7. montelukast or zileuton for LT-dominant phenotype 10 mg PO daily PO daily evening — LT-mediated dominant symptoms (Valent 2020 PMID 33261124) (Counsel montelukast neuropsychiatric warning; zileuton needs LFT monitoring)
8. omalizumab for refractory / idiopathic anaphylaxis 300-600 mg SC q2-4wk SC q2-4wk — Refractory MCAS or idiopathic anaphylaxis (Valent 2020 PMID 33261124; Giannetti 2020 PMID 32276688) (Off-label; durable benefit reported)
9. midostaurin for advanced SM via heme-onc 100 mg PO BID PO BID — Advanced systemic mastocytosis (heme-onc co-management) (FDA approved for aggressive SM, SM-AHN, mast cell leukaemia)

Non-pharmacologic actions:
- Pre-procedure premedication regimen (H1 + H2 + corticosteroid + LTRA) before contrast / anaesthesia / dental procedures (Valent 2020 PMID 33261124)
- Family screening for HαT when proband TPSAB1 duplication confirmed (Lyons 2016 PMID 27749843)
- Symptom + trigger + episode diary for criterion-3 documentation (Valent 2020 PMID 33261124)

AVOID / contraindication checks:
- Never give NSAIDs opioids vancomycin NMBA without anaesthesia and allergy coordination (Valent 2020 PMID 33261124   direct MC degranulating agents)
- Premedicate for contrast and anaesthesia with h1 h2 corticosteroid and leukotriene modifier as needed (Valent 2020 PMID 33261124)
- Montelukast fda 2020 boxed warning neuropsychiatric (screen pre and during)
- Zileuton hepatotoxicity monitor lfts (FDA labelling)
- Midostaurin and avapritinib via heme onc only (advanced SM indication)
- Hereditary alpha tryptasemia uses personalised tryptase reference range (Chovanec 2023 PMID 36170795)
- Never respond to anaphylactic grade event with anti mediator escalation instead of epinephrine

Monitoring

Regimen monitoring:
- symptom trigger episode diary (Valent 2020 PMID 33261124)
- serial basal tryptase to detect clonal progression with HαT corrected reference (Chovanec 2023 PMID 36170795)
- cbc periodic for cytopenia screen
- lft periodic for zileuton or midostaurin treated patients
- montelukast neuropsychiatric screen pre and during (FDA 2020 boxed warning)
- epinephrine auto injector carry expiry technique at every visit

Setting (outpatient) monitoring:
- Symptom diary + therapy response at 4-8 wk intervals
- Serial tryptase with HαT-corrected reference range (Chovanec 2023 PMID 36170795)
- Drug-class safety: montelukast neuropsych; zileuton LFTs; midostaurin GI/hepatic

Follow-up plan: Lifelong chronic-disease maintenance: trigger avoidance + premedication for unavoidable triggers (e.g., contrast studies, surgery, dental procedures — H1 + H2 + cromolyn + corticosteroid premedication regimen), auto-injector + action plan, family screening for HαT, periodic re-evaluation for clonal MC disease emergence.
- Close-out criterion: Lifelong plan documented; premedication template ready for procedures

Monitoring phase: Symptom diary correlating triggers, episodes, mediator measurements, and therapy response. Repeat tryptase periodically to detect clonal MC disease progression. Monitor for therapy side-effects: H2 long-term risk (B12, C. difficile with PPIs if co-prescribed), montelukast neuropsych (FDA 2020 boxed warning), midostaurin GI/hepatic AEs. Auto-injector technique + carry every visit.

Disposition

Current setting: outpatient — Apply the three-criteria diagnostic frame (multi-organ symptoms + objective mediator-release evidence + response to MC-directed therapy), classify primary vs secondary vs idiopathic, recognise HαT, and titrate the stepwise ladder while keeping epinephrine and an action plan in place (Valent 2020 PMID 33261124)

Disposition criteria:
- Continue outpatient allergy follow-up with quarterly review until stable
- Heme-onc referral for elevated tryptase, KIT D816V positive, or refractory disease
- Genetic counselling for confirmed HαT

Escalation triggers (move to higher acuity):
- Active anaphylaxis -> route to allergy.anaphylaxis.v1 for IM epinephrine + supportive
- Basal tryptase >20 ng/mL persistent or rising, cytopenia, hepatosplenomegaly -> haematology for bone marrow + KIT D816V
- Idiopathic anaphylaxis recurrent -> escalate to omalizumab; consider clonal workup

Earlier-Return Triggers

Return-precaution thresholds (watch for):
- [LIFE_THREATENING] Anaphylactic-grade event (two-organ system / CV / airway) in known or suspected MCAS
- [MODERATE] Basal tryptase >20 ng/mL persistent (or HαT-corrected upper limit exceeded), KIT D816V positive, cytopenias, hepatosplenomegaly, lymphadenopathy
- [MODERATE] Planned contrast study, surgery, dental procedure, or anaesthesia in patient with MCAS / clonal MC disease

Citations

- Valent et al 2020 - MCAS Diagnosis, Classification and Management (Int J Mol Sci 2020-11; PMID 33261124) + Giannetti/Akin/Castells 2020 - Idiopathic anaphylaxis as a form of MCAS (J Allergy Clin Immunol Pract 2020-04; PMID 32276688) + Lyons 2016 - TPSAB1 copy-number and hereditary alpha-tryptasemia (Nat Genet 2016-10; PMID 27749843) + Chovanec/Lyons 2023 - personalised tryptase reference ranges (Blood Adv 2023-05; PMID 36170795) [PMID:33261124](https://pubmed.ncbi.nlm.nih.gov/33261124/)
- Cited evidence (PMID 32276688) [PMID:32276688](https://pubmed.ncbi.nlm.nih.gov/32276688/)
- Cited evidence (PMID 27749843) [PMID:27749843](https://pubmed.ncbi.nlm.nih.gov/27749843/)
- Cited evidence (PMID 36170795) [PMID:36170795](https://pubmed.ncbi.nlm.nih.gov/36170795/)

Last reconciled with current guidelines: 2026-05-26.
References
  • Valent et al 2020 - MCAS Diagnosis, Classification and Management (Int J Mol Sci 2020-11; PMID 33261124) + Giannetti/Akin/Castells 2020 - Idiopathic anaphylaxis as a form of MCAS (J Allergy Clin Immunol Pract 2020-04; PMID 32276688) + Lyons 2016 - TPSAB1 copy-number and hereditary alpha-tryptasemia (Nat Genet 2016-10; PMID 27749843) + Chovanec/Lyons 2023 - personalised tryptase reference ranges (Blood Adv 2023-05; PMID 36170795) — PMID:33261124
  • Cited evidence (PMID 32276688) — PMID:32276688
  • Cited evidence (PMID 27749843) — PMID:27749843
  • Cited evidence (PMID 36170795) — PMID:36170795