Mast cell activation syndrome (MCAS) is a condition of inappropriate mast cell release of histamine and other mediators, causing flushing, urticaria, GI symptoms, food sensitivities, fatigue, and sometimes anaphylaxis. Diagnosis combines clinical features with selective lab support (tryptase, urinary metabolites). Treatment is layered: H1 and H2 antihistamines, sometimes cromolyn or ketotifen, leukotriene blockers in selected patients, and identification of triggers. Fishtown Medicine evaluates MCAS in primary care and coordinates with allergy / immunology when needed.
MCAS has become more recognized in Philadelphia in the post-COVID years. Many patients with persistent post-viral symptoms have features consistent with mast cell activation, and many had similar features for years before COVID. The condition is real but the diagnostic criteria are still debated, the workup takes time, and the treatment is largely empirical.
This page is how Fishtown Medicine evaluates and treats MCAS in Philadelphia.
What MCAS looks like
The presentations vary widely. Common features include:
- Flushing of face, chest, neck without obvious trigger.
- Hives or urticaria, sometimes pressure-induced or heat-induced.
- GI symptoms: nausea, cramping, diarrhea, food sensitivities, sometimes mimicking IBS.
- Cardiovascular: palpitations, presyncope, blood pressure swings.
- Respiratory: wheeze, congestion, sometimes mimicking asthma.
- Neurologic: brain fog, headache, sometimes neuropathy.
- Fatigue.
- Multiple medication and food sensitivities.
- Sometimes anaphylaxis or near-anaphylaxis.
Triggers can include foods (often histamine-rich foods, alcohol, fermented products), medications (NSAIDs, opioids, certain antibiotics, IV contrast), heat, cold, stress, exercise, and infection.
MCAS frequently co-occurs with POTS and joint hypermobility (the "MCAS-POTS-EDS" triad), and is common in long COVID populations.
How MCAS is diagnosed
The diagnosis has both consensus and controversy. The 2019 consensus criteria require:
- Typical clinical features of mast cell activation.
- Increase in mast cell mediator (serum tryptase, urinary histamine metabolites, prostaglandin D2) during a symptomatic episode.
- Response to mast-cell-targeted treatment.
The biochemical tests are unreliable in many MCAS patients because the mediator release is episodic. The clinical diagnosis (criteria 1 and 3) is often what we work with in primary care, with biochemical confirmation when feasible.
The workup we run includes:
- Tryptase, ideally during a symptomatic episode but often a baseline.
- 24-hour urine prostaglandin D2, N-methylhistamine, leukotriene E4 when biochemical confirmation is needed.
- CBC, comprehensive metabolic panel, TSH and free T4.
- Iron studies, vitamin D, B12.
- Total IgE to rule out other allergic conditions.
- Sometimes celiac panel and IgG subclasses.
- Evaluation for joint hypermobility, POTS, and long COVID when the picture suggests them.
For patients with elevated baseline tryptase (over about 11.4 ng/mL), evaluation for hereditary alpha-tryptasemia or mastocytosis is warranted.
Treatment: the layered approach
Treatment in MCAS is incremental. The foundation is H1 and H2 antihistamines, layered with additional medications as needed.
Layer 1:
- H1 antihistamines: Loratadine 10 mg, cetirizine 10 mg, or fexofenadine 180 mg, daily or twice daily.
- H2 antihistamines: Famotidine 20 mg twice daily.
Layer 2 (if Layer 1 insufficient):
- Higher antihistamine doses. Up to fexofenadine 180 mg twice daily, cetirizine 10 mg twice daily, sometimes hydroxyzine at bedtime.
- Cromolyn sodium: Oral solution 200 mg four times daily before meals and at bedtime. Reduces GI symptoms most reliably.
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Layer 3 (if persistent symptoms):
- Leukotriene receptor antagonist: Montelukast 10 mg at bedtime.
- Ketotifen (compounded in US): adds H1 antagonism plus mast cell stabilization.
- Aspirin in selected patients (helps prostaglandin-mediated symptoms but contraindicated if NSAID-sensitive).
Layer 4 (refractory):
- Omalizumab (anti-IgE biologic) in selected patients, often in coordination with allergy/immunology.
- Other immunomodulators in coordination with specialty care.
Trigger identification and avoidance (low-histamine diet trial for some patients, careful medication review, environmental factors) is part of the foundation.
How MCAS care works at Fishtown Medicine
First visit is 90 minutes. We build the picture, decide on the workup, and start the foundational treatment immediately. The empirical response to Layer 1 antihistamines is often informative.
Follow-up at 4-6 weeks to review labs, refine treatment, and adjust as needed. Then at 1-3 month intervals.
We coordinate with allergy and immunology when refractory symptoms or unusual features warrant specialty input. Penn and Jefferson both have allergy/immunology programs.
What it costs
Membership at Fishtown Medicine covers all visits and ongoing management; see pricing for current rates. All visits and ongoing management are included. Medications and labs are billed separately. Most foundational MCAS medications (antihistamines, cromolyn, montelukast) are inexpensive at most Philadelphia pharmacies with insurance or cash pricing.
Key Takeaways
- MCAS involves inappropriate mast cell mediator release with a wide symptom spectrum.
- Diagnosis is partly clinical and partly biochemical, with empirical response often confirming.
- Treatment is layered, starting with H1 and H2 antihistamines.
- MCAS frequently co-occurs with POTS, joint hypermobility, and long COVID.
- Fishtown Medicine evaluates and treats MCAS in primary care, coordinating with specialists when needed.
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