What Are the Diagnostic Criteria for MCAS? Consensus-1, Consensus-2, and Where I Stand

If a doctor has told you that you "don't meet the criteria for MCAS," that can feel a lot like being told your symptoms aren't real. Mast cell activation syndrome (MCAS), where mast cells release their chemical messengers too easily, has two main sets of diagnostic criteria, and the two expert groups who wrote them draw the line in different places. So "you don't have MCAS" can mean "you don't meet this group's standard." I'll walk through both sets, why the groups disagree, and how I use them in my own practice.

What are the two sets of MCAS criteria?

The two main proposals are called consensus-1 and consensus-2. The numbering is a little confusing, because consensus-2 was actually proposed first.

Consensus-1: Valent, Akin, and colleagues

Consensus-1 comes from an international group led by Peter Valent and Cem Akin, with experts in allergy, dermatology, hematology (blood disorders), pathology, and molecular medicine. They published the criteria in 2012, and in 2019 they laid out a step-by-step algorithm for using them. The consensus-1 criteria for MCAS need all three of the following:

1. Typical episodes in two or more organ systems at the same time, such as the skin, the gut, the lungs, or the heart and blood pressure. In a 2020 review, the consensus-1 group described these episodes as usually meeting the criteria for anaphylaxis, which is a severe whole-body reaction.
2. Lab evidence that mast cells released their contents during an episode. Their preferred marker is tryptase, an enzyme mast cells store. Under consensus-1, a flare tryptase has to rise more than 20% above your own calm-day baseline, plus 2 ng/mL (nanograms per milliliter). With a baseline of 10 ng/mL, the flare result has to be above 14. A clear rise in another mast cell chemical can also count.
3. Symptoms that improve on medicines that block mast cell chemicals or calm the mast cells down.

Timing is the hard part of consensus-1, since tryptase rises and falls within a few hours. British anaphylaxis guidelines, summarized in a 2019 review by Beck and colleagues, recommend drawing the flare sample ideally 1 to 2 hours after symptoms start and no later than 4 hours. The consensus-1 group says the baseline gets drawn at least 24 to 48 hours after symptoms have completely gone away. A lot of people I see with MCAS flare and calm down all the time, so their tryptase may never get back to a true baseline, and that blurs the line between a baseline and a flare. Our post on MCAS testing goes through the tryptase and urine tests in more detail.

Consensus-2: Molderings, Afrin, and colleagues

Consensus-2 started as a 2011 proposal by Gerhard Molderings, Lawrence Afrin, and colleagues, and its authors revised it in 2016 and again in 2017. In 2020, a larger group of clinicians and researchers from many specialties, with Afrin as lead author, gave it the name consensus-2 and restated the 2017 version. The consensus-2 criteria for MCAS ask for a typical pattern of mast cell symptoms, which can affect almost any part of the body, plus at least one supporting finding from this longer list:

1. Extra or abnormal mast cells on a biopsy, from the bone marrow or another organ such as the gut or the urinary tract. The consensus-2 authors count 20 or more mast cells in one high-power microscope field as extra, and abnormal means an unusual spindle shape or unusual markers on the cell surface.
2. Certain genetic changes in mast cells that have been shown to make them more active.
3. An above-normal level of a mast cell chemical in the blood or urine, such as tryptase, histamine or its breakdown product N-methylhistamine, heparin, chromogranin A, prostaglandin D2, or leukotriene E4.
4. A response to medicines that block mast cell activation or the chemicals mast cells release. The consensus-2 authors added this in their 2016 update, because testing for mast cell chemicals is hard to get in many parts of the world, so a response to treatment can count when the lab values are normal. A 2025 paper, whose corresponding author is one of the consensus-2 authors, notes that most clinicians who use consensus-2 would still rather have a lab result.

There is a trade-off in that longer list of tests. Histamine, prostaglandins, and leukotrienes can come from other cells too, so on their own they can't prove the mast cells made them. Tryptase is more specific to mast cells, which is a big reason consensus-1 prefers it. Chromogranin A has its own confounders, and the consensus-2 authors note that heart or kidney failure, neuroendocrine tumors, chronic atrophic gastritis, and recent use of acid-reducing proton pump inhibitors can also raise it.

Why do the two groups disagree?

The disagreement comes down to which mistake each group sees as the bigger harm.

The consensus-1 group worries about overdiagnosis. In a 2021 review, Gülen, Akin, Valent, and colleagues wrote that a growing number of people are given an MCAS diagnosis based on nonspecific criteria, and that this can sometimes delay recognition and treatment of the condition a person actually has, which often turns out to be unrelated to mast cells. The group's 2020 review, led by Valent, adds that criteria built only on symptoms can't reliably separate MCAS from the conditions that look like it.

The consensus-2 group worries about underdiagnosis. In their 2020 paper, Afrin and colleagues agreed that overdiagnosis by consensus-2 has the potential to be a problem. They argued that underdiagnosis by consensus-1 is the far larger one, partly because most people with MCAS can eventually find a treatment that gives them lasting improvement. They concluded that until careful research gives clearer answers, a diagnosis by either set is "valid, reasonable, and helpful."

The numbers change a lot depending on which set is used. One pediatric cardiology practice in California reviewed the records of 100 young people with POTS (postural orthostatic tachycardia syndrome, where the heart races on standing). Only 69 of them were able to get tryptase tested at all. Using a loosened version of the consensus-1 criteria for MCAS that counted a single high tryptase, 2 of those 69 qualified, and none met the published consensus-1 requirement of a baseline plus a flare tryptase. Using symptoms and response to treatment, 87 of the 100 had MCAS. The lead clinician is one of the consensus-2 authors, which the paper states openly, and our post on MCAS in POTS and hEDS goes through these numbers in more depth.

Consensus-2 isn't the set the main allergy and mastocytosis organizations use. A 2025 Canadian review describes the consensus-1 criteria as coming from an international consortium of mast cell experts endorsed by major scientific organizations. The American Academy of Allergy, Asthma & Immunology published its own criteria in a 2019 work group report, with the stated aim of not overdiagnosing people who likely have other conditions. So a lot of allergists go by the stricter rules, and it helps to know which set your doctor uses.

Where do I stand?

I come at this patient first. MCAS is real, and the people I see with it aren't making anything up. Many of them have collected several separate diagnoses, like IBS, anxiety, or fibromyalgia, before anyone connected the pieces. Being heard is where care starts.

When someone comes to me with an MCAS picture, I do the workup properly first, which means checking for the conditions that can look like MCAS before I call it MCAS. Once the workup has ruled those out and MCAS fits the picture, I diagnose by consensus-2. In my 2025 diagnosis lecture for clinicians, I said consensus-2 offers the best model for real-world practice, because it uses symptom patterns, supportive labs when they can be had, and a trial of treatment. I still think so. A normal tryptase doesn't rule MCAS out, and even a 2025 review written around the consensus-1 criteria says that normal values don't exclude the diagnosis. In my experience, a good response to treatment can tell me as much as a positive lab test.

I'm also comfortable trying treatment before a formal diagnosis when the picture points to mast cells. The current health system usually needs a diagnosis before insurance will cover treatment, which is one reason the label makes such a difference for my patients. Even the consensus-1 group wrote in their 2020 review that in some cases, initial therapy has to start before a definitive MCAS diagnosis can be made.

What I won't do is send someone home with "you don't meet the criteria" and nothing to try. Gatekeeping a diagnosis when nothing else is offered harms people.

What if you don't meet the criteria?

Even the stricter consensus-1 group writes about people who fall outside their criteria. In their 2020 review, Valent and colleagues described mast cells acting in just one organ, like hives in the skin or asthma in the lungs. Activity in a single organ can't meet the consensus-1 criteria for MCAS, since those criteria need more than one organ system, yet the authors wrote that these cases can still need mast cell stabilizers or medicines that block mast cell chemicals. They also wrote that symptoms like headache, fatigue, nausea, and poor sleep fall outside their MCAS criteria, and that these symptoms are clinically relevant and should still be treated for relief.

So missing the criteria doesn't mean nothing can be done. You might have a different diagnosis, mast cell activity in one organ, or MCAS that hasn't been confirmed on a lab test yet, and each of those can still come with something to try.

What else can look like MCAS?

Both groups want the look-alikes checked. The consensus-2 authors wrote in 2011 that the relevant differential diagnoses should be excluded, and their list of conditions that can look like MCAS includes carcinoid tumors, pheochromocytoma (an adrenal tumor that releases adrenaline-type hormones), thyroid disorders, celiac disease, eosinophilic gastroenteritis (gut inflammation driven by a different immune cell), and hereditary angioedema. The consensus-1 group's 2020 list of MCAS look-alikes covers broader categories: infections, autoimmune disease, heart, hormone and nerve conditions, psychiatric conditions, and poisonings. A lot of the look-alikes are easier to test for than MCAS is, with one blood test, a urine test, or a scan.

Having MCAS doesn't rule out a second condition either. The consensus-1 group gave a hypothetical example of a narrowed heart valve plus MCAS, where both can cause low blood pressure and fainting, and wrote that the MCAS criteria shouldn't exclude someone for having another disease.

A high baseline tryptase is its own question. Hereditary alpha-tryptasemia (HαT) is an inherited trait, described by Lyons and colleagues in 2016, in which extra copies of the TPSAB1 gene raise the baseline tryptase. A high baseline raises the question of HαT, and a TPSAB1 genetic test answers it. The consensus-1 group suggests considering that test for anyone whose tryptase comes back at 8 ng/mL or higher. Hereditary angioedema, from the look-alike list above, is a separate condition with a similar name.

Who can diagnose MCAS?

Any clinician who knows the MCAS criteria and can order the tests can make the diagnosis. I'd start with your primary care provider and ask whether they know much about MCAS and whether they're willing to learn. A lot of primary care doctors are really busy, so if they can't take it on, an allergist or rheumatologist who's familiar with MCAS is the next step. Between those two, an allergist is probably more familiar with the criteria and the look-alikes.

I'm a naturopathic doctor, so I have some bias toward naturopathic doctors, who are trained to look across all your systems instead of one. The rules for naturopathic doctors differ from state to state, though, so I'd still start with primary care. The diagnosis can take several visits, because the look-alikes get ruled out one at a time, which is why it helps a lot to see someone you can keep going back to.

Questions to bring to your next MCAS appointment

1. Which MCAS criteria do you use, consensus-1 or consensus-2?
2. Can I have a standing lab order for a flare tryptase, plus a calm-day baseline?
3. Which look-alikes have we ruled out so far, and which are still open?
4. If my baseline tryptase is high, does a TPSAB1 test make sense?
5. If I don't meet the criteria, what can we try in the meantime?

The MCAS hub collects the rest of our diagnosis posts.

If you're living with MCAS and want a structured starting point, MCAS: What You Need to Know First walks through what the diagnosis actually means, where to begin, and what to do when diet alone isn't enough.

FAQ

What are the diagnostic criteria for MCAS?

There are two main sets. The consensus-1 criteria for MCAS, from Valent, Akin, and colleagues, need all three of episodes in two or more organ systems, a lab rise during an episode (preferably tryptase), and a response to mast cell medicines. The consensus-2 criteria for MCAS, from Molderings, Afrin, and colleagues, accept a typical symptom pattern plus at least one supporting finding from a longer list, and since a 2016 update that list includes a response to treatment, which can count when the labs are normal.

What is the difference between consensus-1 and consensus-2?

Consensus-1 is stricter and leans on a tryptase rise during a flare, because its authors worry that with broader criteria, other conditions get mistaken for MCAS and diagnosed late. Consensus-2 accepts more kinds of evidence, because its authors see underdiagnosis as the bigger harm. The consensus-2 authors wrote that, until better research exists, a diagnosis by either set is valid.

Can you have MCAS with a normal tryptase?

Yes. Consensus-1 looks for a rise above your own baseline during a flare, so a normal calm-day tryptase can still fit, and a 2025 review written around the consensus-1 criteria says normal values don't exclude the diagnosis. Consensus-2 also accepts other markers and a response to treatment.

Can MCAS be treated without a diagnosis?

It can be. I'm comfortable trying treatment before a formal diagnosis when the picture points to mast cells, and even the consensus-1 group wrote in 2020 that initial therapy sometimes has to start before a definitive MCAS diagnosis can be made. The current health system usually needs a diagnosis before treatment gets covered.

What kind of doctor diagnoses MCAS?

Any clinician who knows the MCAS criteria and can order the tests can make the diagnosis. I'd start with your primary care provider. If they can't take it on, an allergist or rheumatologist who knows MCAS is the next step, and between those two, an allergist is probably more familiar with the criteria.

References

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