Lately, more and more patients are coming in asking about mast cell activation syndrome. Maybe they’re experiencing flushing, tachycardia, diarrhea, wheezing, or fainting. Maybe MCAS came up alongside POTS (postural orthostatic tachycardia syndrome) or Ehlers-Danlos syndrome on their social media feed. Maybe they tried an antihistamine and felt better. And now they’re asking: “Do I have MCAS?”
MCAS has become increasingly visible online, while the actual diagnosis remains uncommon when rigorous criteria are applied. That puts primary care clinicians in a tricky (but familiar) position: taking a patient’s symptoms seriously without prematurely attaching a rare diagnosis to a broad collection of complaints.
So what does mast cell activation syndrome actually look like, and what should we do when a patient asks us to evaluate them for it?
Mast cell activation syndrome, or MCAS, is characterized by recurrent episodes of severe mast cell mediator release affecting at least two organ systems. According to the American Academy of Asthma, Allergy, and Immunology (AAAAI), diagnosis requires: severe symptoms, objective evidence of mast cell activation, and improvement with mast cell–directed treatment.
That definition matters because mast cell activation is not the same thing as MCAS.
Mast cells are immune cells produced in the bone marrow and found throughout the body. When activated, they release many different kinds of mediators, including histamine. Since those mediators affect many different organ systems, it makes sense that mast cell activation can cause symptoms throughout the body.
MCAS causes recurrent, discrete episodes of severe symptoms involving at least two organ systems. Patients may experience skin symptoms like flushing, urticaria (hives), angioedema, or pruritus; cardiovascular signs such as hypotension, syncope, or tachycardia; respiratory symptoms like shortness of breath, wheezing, dyspnea, or nasal congestion; and abdominal pain, diarrhea, nausea, or vomiting when the GI tract is affected.
Potential triggers include heat, alcohol, exercise, infection, medications, stress, hormonal fluctuations, pressure, or friction. The important distinction is the pattern: symptoms should be episodic, systemic, and significant. Urticaria alone is not MCAS. Neither is chronic bloating or isolated tachycardia.
That distinction can be especially helpful when a patient arrives with a long symptom list gathered from social media. Rather than asking whether each individual symptom could occur with MCAS (the answer may often be yes), ask whether the overall pattern fits recurrent systemic mast cell activation.
This is where things get complicated. There is no universal agreement about exactly how MCAS should be defined, and different diagnostic frameworks can produce very different estimates.
Using the more stringent AAAAI criteria, MCAS appears to be uncommon. In one retrospective study of 703 patients evaluated for suspected idiopathic MCAS, only 4.4% ultimately met the study’s diagnostic criteria. However, another global working group argues for broader diagnostic criteria that do not require the same biochemical evidence of mast cell mediator release.
That disagreement has fueled an ongoing debate about whether MCAS is being underdiagnosed, over-attributed, or both. In primary care, we don’t need to settle that debate in the exam room. What we do need is a consistent diagnostic approach.
A diagnosis of MCAS requires 3 things:
Symptoms + laboratory evidence + treatment response.
The patient should have recurrent severe episodes affecting at least two organ systems.
Testing should show evidence of mast cell mediator release during an episode; this is most easily done with serum tryptase levels, which should be elevated on two separate occasions to make the diagnosis.
Symptoms should improve with treatment targeting mast cell mediators, such as antihistamines.
Serum tryptase is one of the most accessible tests used to document mast cell activation, but timing is everything. To make the diagnosis, the patient needs to have a normal baseline tryptase level plus at least 2 elevated tryptase levels drawn during symptom flares.
An acute tryptase should be drawn shortly after symptoms begin — roughly within 30 minutes to 2 hours, although a sample obtained within 4-6 hours may still be informative. Then compare that result with a baseline tryptase drawn after symptoms resolve, at least 24 hours later.
Baseline tryptase matters for another reason, too. A persistently elevated baseline level can point toward other conditions, including hereditary alpha-tryptasemia or mastocytosis, rather than MCAS itself.
Treatment focuses on blocking the effects of mast cell mediators and avoiding known triggers.
Nonsedating H1 antihistamines are commonly used first and may be given 2-4 times the usual daily dosage. H2 blockers such as famotidine may be helpful when GI symptoms are prominent, and leukotriene receptor antagonists such as montelukast are another option. More persistent or severe symptoms may require therapies such as cromolyn sodium or specialist-directed treatment.
Identifying triggers can also help. If episodes consistently occur after alcohol, heat, exercise, or another exposure, reducing that trigger may improve symptoms while the diagnostic evaluation continues.
Patients with significant systemic reactions should also have an emergency plan, which may include an epinephrine auto-injector, albuterol rescue inhaler, and supine positioning instructions for hypotension.
Importantly, treatment response is part of the diagnostic picture, but improvement with an antihistamine alone does not prove MCAS. Plenty of mast cell–mediated symptoms outside of MCAS can respond to the same medications.
This may be one of the most important conversations we have with patients: mast cell activation can occur without mast cell activation syndrome.
Allergies, infections, heat, stress, medications, and other conditions can activate mast cells. A patient can therefore have real symptoms influenced by histamine or other mast cell mediators without meeting diagnostic criteria for MCAS.
A negative MCAS workup shouldn’t translate to, “Nothing is wrong with you.” A better next question is: “If this isn’t MCAS, what else could be driving these symptoms?”
That keeps us curious without forcing every complaint into one diagnostic box.
Both perimenopause and long COVID have become part of the MCAS conversation online.
Hormonal fluctuations are recognized potential triggers for mast cell activation. Mast cells express estrogen and progesterone receptors, and laboratory research suggests estradiol can affect mast cell behavior. But that does not establish that perimenopause causes MCAS or that the syndrome becomes substantially more common during the menopausal transition. For now, that connection remains an area where more research is needed.
Long COVID is another evolving area. Studies have reported mast cell activation–type symptoms among people with long COVID, and investigators continue to explore whether mast cells contribute to its pathophysiology. A history of COVID may make mast cell activation worth considering when the symptom pattern fits, but it should not lower the diagnostic bar for MCAS.
When a patient asks, “Could I have MCAS?” you don’t need to solve the entire case in one visit. A reasonable starting plan is:
Clarify whether symptoms occur in discrete, severe episodes involving at least two organ systems, and look for alternative explanations.
Obtain a baseline serum tryptase while the patient is asymptomatic, and place a standing order for acute tryptase during a typical flare.
After appropriate testing, consider a trial of mast cell–directed therapy and assess whether symptoms meaningfully improve.
Refer to allergy/immunology when the diagnosis remains unclear, symptoms are refractory to initial treatment, or baseline tryptase is persistently elevated.
When a patient arrives asking whether they have MCAS, we don’t have to choose between dismissing the diagnosis and immediately embracing it.
There’s a better middle ground: listen carefully, look for the characteristic pattern of recurrent systemic episodes, pursue objective testing when possible, keep other diagnoses on the table, and treat the symptoms you can.
To hear more on this topic, listen to Primary Care RAP.
References
Weiler CR, Austen KF, Akin C, et al. AAAAI Mast Cell Disorders Committee Work Group report: mast cell activation syndrome (MCAS) diagnosis and management. J Allergy Clin Immunol. 2019;144(4):883-896. doi:10.1016/j.jaci.2019.08.023. PMID: 31476322
Zaghmout T, Maclachlan L, Bedi N, Gülen T. Low prevalence of idiopathic mast cell activation syndrome among 703 patients with suspected mast cell disorders. J Allergy Clin Immunol Pract. 2024;12(3):753-761. doi:10.1016/j.jaip.2023.11.041. PMID: 38056692