What Test Can Confirm MCAS? Tryptase, Urine Mediators, and When to Test
What the tryptase and urine tests for MCAS measure, and when to have them drawn.
Photo: Debby Hudson / Unsplash
No single test can confirm MCAS (mast cell activation syndrome) on its own. The lab portion of the diagnosis looks for a rise in mast cell chemicals during a flare, compared with your own level on a calm day, so the timing of the draw is usually more important than the name of the test on the order. If someone read your normal tryptase as ruling MCAS out, that result can still fit the diagnosis.
What do the MCAS criteria ask for?
MCAS means mast cells, the immune cells that store histamine and other chemical messengers, release those messengers too easily. In 2012, an international working group led by Peter Valent published consensus criteria built on three parts: typical symptoms, a substantial short-lived rise in serum tryptase or in other mast cell mediators or their breakdown products in urine, and improvement in symptoms with medications that block mast cell mediators or their effects.
In practice, I find that an MCAS diagnosis is a criteria-based process. You and your doctor match symptoms and triggers, then see whether a trial of mast cell medication helps, which can take some trial and error. Other clinicians have proposed broader alternative criteria, and in a 2021 review Gülen and colleagues compared the published proposals and argued for applying the consensus criteria. Our post on MCAS in POTS and hEDS shows how much the choice of criteria changes the numbers.
Why can your tryptase be normal if you have MCAS?
Tryptase is an enzyme stored in mast cells, and a 2026 review by De Amici and colleagues described it as the principal blood marker of mast cell number and activity. In the patients I see with MCAS, the calm-day tryptase is usually in the normal range. The criteria compare a flare level against your own baseline, so the calm-day number is the reference point a flare result gets measured against.
The consensus formula, usually called the 20% + 2 rule, counts a flare result as a meaningful rise when it is above 1.2 times your baseline plus 2 ng/mL (nanograms per milliliter). With a baseline of 5 ng/mL, the flare result would need to be above 8 ng/mL. The same consensus group proposed it in 2012, and Valent's group reported in 2019 that independent groups have since validated it.
Catching the right moment for a flare draw can be hard. In their 2019 review of anaphylaxis, which is a severe allergic reaction, Beck and colleagues explained that tryptase in the blood is highest about one to two hours after the reaction starts, and half of it is gone roughly every two hours after that. The British anaphylaxis guidance they summarized recommends a first sample as soon as possible, a second ideally within one to two hours of symptoms starting and no later than four hours, and a baseline at least 24 hours later. In my practice, I use the same windows for a flare draw. During the worst flares, everyone can be focused on getting you stable rather than on labs.
It's a good idea to ask your doctor about a standing lab order, which lets you go in for a tryptase during a flare without booking a new visit. Some people I see never get back to a true baseline, because one trigger after another keeps their mast cells active (the bucket model explains how triggers add up), and that can make the tryptase harder to catch.
What can urine mediator tests add?
Urine tests measure breakdown products of three mast cell mediators: N-methylhistamine (from histamine), leukotriene E4, and 2,3-dinor-11β-prostaglandin F2α (from prostaglandin D2). In a 2026 review, Mayo Clinic allergists Voelker and Pongdee pointed out that serum tryptase has an ideal collection window and needs a blood draw. Writing about anaphylaxis, they described urine mediators collected during an acute reaction and compared with a baseline sample as emerging markers, and they said further research is needed to validate them in anaphylaxis. With that comparison in mind, I would rather see a 24-hour urine collected on a day with symptoms than on a good day.
These markers have limits as well. Butterfield and Weiler noted in a 2020 Mayo review that cells other than mast cells have been reported as sources of these mediators, although in systemic mastocytosis they considered mast cells the main source. For that reason, I read a raised urine result as supportive rather than as proof on its own that mast cells were the source. Beck's group listed N-methylhistamine as a marker that can be raised in a variety of non-allergic conditions and when bacteria contaminate the urine sample. Valent's consensus group wrote in 2019 that a substantial rise in tryptase over a person's own baseline is the most specific and reliable marker of systemic mast cell activation.
What if your baseline tryptase is always high?
There is an inherited trait called hereditary alpha-tryptasemia (HαT), in which extra copies of the gene for alpha-tryptase raise the baseline tryptase. Estimates of how common it is vary: Valent's group put it at about 3% to 5% of healthy people in 2019, and Vazquez and colleagues at 5% to 7% of Western populations in 2022.
In a 2022 study from a regional US health system, Waters and colleagues looked at 58 people whose baseline tryptase was 11.5 ng/mL or higher. The team found HαT in 37 of them (63.8%), chronic kidney disease in 7, and a clonal myeloid disorder, a group of bone marrow conditions, in 12, with some people in more than one group. Among people with a raised baseline and more than one measurement, 41.5% had baseline readings that varied by more than the 20% + 2 formula. For them, a single flare result can be harder to read.
When the baseline is raised, HαT or another cause might be the reason rather than MCAS, and a raised baseline doesn't rule MCAS out either, since the 20% + 2 formula measures the rise over each person's own baseline. Tryptase genotyping, a genetic test for the extra gene copies, can identify HαT, and it might be helpful to make sure the clinician reading your results knows whether HαT is part of the picture.
What can you bring to your next appointment?
Three things you could ask your doctor about:
- Whether you can have a standing order for a flare tryptase, plus a calm-day baseline.
- Timing a 24-hour urine collection to a day when you have symptoms.
- HαT testing, if your baseline tryptase is high every time it is checked.
If your workup included a DAO (diamine oxidase) test, our post on DAO testing covers what a low result can and can't tell you, and the MCAS hub collects the rest of the 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
Can you have MCAS with a normal tryptase?
Yes. In the patients I see with MCAS, the calm-day tryptase is usually in the normal range. The consensus criteria look for a rise above your own baseline during a flare, so a normal calm-day result can still fit the diagnosis.
What is the 20% + 2 rule for tryptase?
The consensus formula counts a flare tryptase as a meaningful rise when it is above 1.2 times your baseline plus 2 ng/mL. With a baseline of 5 ng/mL, for example, the flare result would need to be above 8 ng/mL.
How soon after a flare should tryptase be drawn?
In anaphylaxis, tryptase in the blood is highest about one to two hours after the reaction starts. British anaphylaxis guidance summarized in a 2019 review recommends a first sample as soon as possible, a second ideally within one to two hours of symptoms starting and no later than four hours, and a baseline at least 24 hours later.
Is a urine test better than a tryptase for MCAS?
Each test answers a slightly different question. Urine mediators need no blood draw and can be collected around a symptomatic day, while Valent's consensus group considers a rise in tryptase over your own baseline the most specific marker of systemic mast cell activation. A Mayo Clinic review from 2026 described urine mediators as emerging markers that still need further research to validate in anaphylaxis.
What is hereditary alpha-tryptasemia?
Hereditary alpha-tryptasemia (HαT) is an inherited trait in which extra copies of the alpha-tryptase gene raise the baseline tryptase. Published estimates range from about 3% to 7% of people, and a genetic test can identify it.
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