Mast Cell Activation Syndrome (MCAS): Symptoms, Testing, and Treatment Options
If you've been dealing with unexplained flushing, hives, stomach problems, or episodes of feeling unwell, you may have come across the term Mast Cell Activation Syndrome (MCAS).
And if you've spent any time researching MCAS online, you've probably noticed something: there is a LOT of conflicting information out there.
Some sources suggest that MCAS could explain nearly every unexplained symptom. Others make it seem like the condition doesn't exist at all.
The truth is somewhere in between.
At Inspire Allergy & Asthma, we understand how frustrating it can be to experience symptoms without clear answers. We also believe that you deserve an evaluation based on the best available scientific evidence, not guesswork.
Let's talk about what MCAS actually is, how allergists evaluate it, and what treatment options may help.
What Is Mast Cell Activation Syndrome (MCAS)?
Mast cells are immune cells found throughout your body, particularly in your skin, respiratory tract, and gastrointestinal system.
They're an important part of your immune system and help protect you from infections and other threats.
When activated, mast cells release substances called mediators, including histamine, tryptase, leukotrienes, and prostaglandins.
These mediators can cause symptoms such as itching, flushing, swelling, and changes in breathing or blood pressure.
Normally, mast cells activate when they're supposed to. But sometimes, mast cells release their mediators inappropriately or excessively.
Mast Cell Activation Syndrome (MCAS) is a condition involving recurrent episodes of systemic mast cell activation that meet specific clinical and laboratory criteria.
Importantly, having symptoms associated with mast cell activation does not automatically mean you have MCAS.
What Are the Symptoms of MCAS?
Because mast cells are found throughout the body, their activation can affect multiple organ systems.
Symptoms may include:
Skin: Flushing, hives, itching, or swelling
Gastrointestinal: Abdominal cramping, diarrhea, nausea, or vomiting
Respiratory: Wheezing, coughing, or difficulty breathing
Cardiovascular: Lightheadedness, rapid heartbeat, low blood pressure, or fainting
Some patients also report fatigue, brain fog, headaches, and other nonspecific symptoms.
However, these symptoms alone are not enough to diagnose MCAS.
Under widely accepted international consensus criteria, MCAS involves recurrent, significant episodes affecting at least two organ systems, often resembling anaphylaxis.
Many other medical conditions can cause similar symptoms, which is why a thorough evaluation is so important.
How Is MCAS Diagnosed?
This is where things can get a little complicated.
Unfortunately, there is no single blood test that can definitively diagnose or exclude MCAS in every patient.
The widely accepted consensus approach involves three important components:
1. Recurrent symptoms consistent with systemic mast cell activation
These typically involve at least two organ systems during an episode, such as flushing accompanied by significant gastrointestinal symptoms or low blood pressure.
2. Objective evidence of mast cell mediator release
This may involve measuring serum tryptase during an acute episode and comparing it with a baseline measurement.
The established tryptase criterion is an event-related increase greater than 20% above the patient's baseline plus 2 ng/mL.
For certain patients, urinary mast cell mediators may also provide supportive information.
3. Improvement with medications targeting mast cell mediators
These might include antihistamines or other medications that reduce the effects of mast cell mediators.
All three components are important when applying the standard consensus diagnostic criteria.
It is also worth recognizing that experts continue to study and debate aspects of MCAS diagnosis. This is an evolving area of medicine, and we believe in being transparent about what is established and what remains uncertain.
What Tests Are Used to Evaluate MCAS?
Depending on your symptoms and medical history, testing may include:
Serum tryptase: A blood test that can help identify mast cell activation, particularly when measured during a significant episode and compared with your baseline.
Urine N-methylhistamine: A metabolite of histamine that may provide additional information about mediator release.
Urine prostaglandin D2 metabolites: Certain urinary prostaglandin metabolites may help support the evaluation.
Urine leukotriene E4: Another mediator that may be evaluated in selected patients.
These tests have limitations. Some are influenced by collection timing, handling, medications, and conditions unrelated to MCAS.
An isolated abnormal urine mediator result does not necessarily confirm MCAS. Similarly, a normal baseline tryptase does not automatically rule out episodic mast cell activation.
We also look for other possible explanations for your symptoms, including allergic conditions, chronic spontaneous urticaria, medication reactions, and certain gastrointestinal or cardiovascular conditions.
Our goal is not simply to order every available test. It's to determine which tests are appropriate, interpret them correctly, and use those findings to develop a meaningful treatment plan.
Can MCAS Be Treated?
Yes. Treatment is individualized based on symptoms, the underlying cause of mast cell activation, and the results of your evaluation.
Depending on the situation, treatment options may include:
H1 antihistamines
Medications such as cetirizine, fexofenadine, and other antihistamines may help reduce symptoms related to histamine, including itching, hives, and flushing.
H2 antihistamines
Medications such as famotidine may be helpful for selected symptoms, particularly some gastrointestinal complaints.
Leukotriene modifiers
Medications such as montelukast may be considered in certain situations, depending on symptoms and potential risks.
Mast cell stabilizing medications
Cromolyn sodium may be considered in selected patients, particularly when gastrointestinal symptoms are prominent. The evidence supporting its use specifically in MCAS remains limited.
Biologic therapies
For selected patients with recurrent or difficult-to-control mast cell-mediated symptoms, omalizumab (Xolair) may be considered by a specialist.
Xolair is approved for certain allergic conditions, but its use specifically for MCAS is off-label, and the evidence is more limited than it is for its approved indications.
For patients at risk of anaphylaxis, we also discuss whether an epinephrine auto-injector and emergency action plan are appropriate.
Treatment should be tailored to the individual rather than following a one-size-fits-all protocol.
What About Low-Histamine Diets and Supplements?
If you've researched MCAS, you've probably come across lengthy lists of foods to avoid, supplements to purchase, and lifestyle changes to make.
Unfortunately, the evidence supporting many of these recommendations is limited.
There is currently insufficient high-quality evidence that a strict low-histamine diet is an effective treatment for MCAS.
Some individuals may identify reproducible food triggers, and we can certainly discuss those. However, unnecessarily restricting large numbers of foods can lead to nutritional deficiencies, increased anxiety around eating, and a significant decrease in quality of life.
Similarly, many supplements marketed for mast cell stabilization have not been adequately studied in patients with MCAS.
We believe treatment recommendations should be guided by evidence, safety, and your individual symptoms, not by the latest trend circulating on social media.
Why Seeing an Allergist Can Help
One of the most challenging parts of suspected MCAS is that symptoms can overlap with many other conditions.
As a board-certified allergist and immunologist, I evaluate conditions involving mast cell activation, including chronic urticaria, anaphylaxis, allergic disorders, and suspected MCAS.
During your visit, we'll take the time to review your symptoms, identify patterns, discuss previous evaluations, and determine whether additional testing is appropriate.
And sometimes, the most helpful outcome is discovering that a different condition is responsible for your symptoms, especially if it leads us toward a more effective treatment.
You Deserve to Be Heard
One thing I want patients to understand is that not having a diagnosis does not mean your symptoms aren't real.
Living with unpredictable symptoms can be exhausting. It's frustrating to feel like you're constantly searching for answers, trying different medications, or wondering when the next episode might happen.
We understand that.
At Inspire Allergy & Asthma, we're here to listen, support you, and work through the evaluation together.
I also believe it's important to be honest about the limits of our medical knowledge. MCAS is an area of active research, and while we have established diagnostic guidelines, there are still questions we're working to answer.
You deserve a physician who takes your symptoms seriously while also being thoughtful about testing, diagnosis, and treatment.
Looking for an MCAS Evaluation in Indianapolis or Carmel?
If you're experiencing symptoms concerning for mast cell activation syndrome or have previously been told you might have MCAS, we're happy to help you explore the next steps.
At Inspire Allergy & Asthma, we offer longer appointments and individualized care for patients in Carmel, Indianapolis, Westfield, Zionsville, Fishers, and surrounding communities.
Our direct-care model allows us to spend time reviewing your history, explaining testing options, and developing a plan together.
You don't have to navigate these complicated symptoms alone. We're here to help.
Visit www.inspireallergyandasthma.com to learn more or schedule an appointment.
Anita Sivam, DO, FAAAAI
Board-Certified Allergist & Immunologist
Founder, Inspire Allergy & Asthma
This article is for educational purposes and does not replace individualized medical advice. Symptoms of anaphylaxis, including significant breathing difficulty, throat swelling, or faintness with a suspected allergic reaction, require immediate emergency treatment.
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