Mast Cell Activation Syndrome (MCAS) Diagnosis

Woman suffering from symptoms.

“When I don’t eat, I don’t have symptoms.”

“I’m sensitive to everything – smells, supplements, foods, you name it!”

“My rashes/hives just appear without a trigger – dermatologists can’t figure out how to help me.”

“I can only eat a few foods.”

“I get sinus infections a lot, and have to use antibiotics to get rid of them.”

These are some of the comments I hear from patients in whom I suspect Mast Cell Activation Syndrome (MCAS).

Testing for MCAS is not very accurate, so we have to rely on a collection of symptoms. There are a lot of symptoms associated with MCAS. A lot! Anyone can develop “medical student syndrome” just by reading through these lists (especially the list found on this site)! But many of the symptoms in MCAS can be attributed to other conditions!

Instead of reading through the list of symptoms, however, I recommend starting with the questionnaire below. It is based off the questionnaire found in Dr. Lawrence Afrin’s 2014 article, “A concise, practical guide to diagnostic assessment for mast cell activation disease.” I just simplified the language in the list, and categorized the questions differently (to make it easier to fill out)!


Mast cells are part of our immune system, and release over 200 chemicals like histamine and heparin when responding to a potential threat in the body. In the right quantity, these chemicals are anti-inflammatory and increase blood flow (to eliminate infections, allergens, or inflammation). When too many of these chemicals are released, we are more likely to experience symptoms (the most common ones that most people know of are allergy symptoms like runny nose and itchy eyes). However, having allergies does not mean that you have MCAS!

Mast cells are found all over the body, so MCAS can affect almost any organ or tissue. Some people have more gut symptoms, others experience more respiratory symptoms; some have more skin issues, others have more cognitive issues. And anyone with MCAS can have symptoms that affect multiple areas of the body.

In my medical education, we learned about a rare condition known as mastocytosis, which is basically an accumulation of mast cells in various ares of the body, mostly due to a genetic mutation. But we never learned about MCAS, which is a collection of symptoms that involves “hyperactive” mast cells (without excessive accumulation in various tissues).

Unfortunately, MCAS is not often on the “radar” of healthcare practitioners so it is often missed (especially when cognitive or gut symptoms are the primary issues).


There are some imperfect ways to test for MCAS, which involve blood tests, urine tests, or tissue biopsies. But most diagnoses are based off medical history and the collection symptoms, as well as the response to treatments.

Why is it so difficult to receive a diagnosis from tests?

  • First, most of the testing occurs when someone either doesn’t have symptoms, or in the middle of an episode of symptoms.
  • And second, the chemicals released by mast cells are only circulating in the body (i.e. testable) for a short time – often only a few minutes! Thus, laboratory diagnosis can be difficult to obtain.

However, the symptoms of MCAS are often attributable to other illnesses, so I don’t recommend considering MCAS as a diagnosis based solely off a list of symptoms. I’ve unfortunately seen patients (and my colleagues in functional medicine) falsely self-diagnose just off a list of symptoms.

The following questionnaire can help, but response to treatments are something to add to the “diagnosis”. Luckily, most treatments can be purchased over the counter (but the timing and dosing of these matters, which is why I recommend appropriate guidance from a medical or nutrition professional).


According to Dr. Lawrence Afrin, MCAS is likely under-diagnosed and more prevalent than we originally thought – up to 14% of the population in America may have it.

The following questionnaire is one of the more validated ways to diagnose suspected MCAS. It is based off symptoms, medical history, and test results. It will take 5-10 minutes to complete.

We must remember that MCAS is still a poorly understood condition, and information is constantly evolving. Right now, we don’t have good tests to definitively diagnose MCAS.

At the end of the day, however, a diagnosis is just a label that should guide treatments. If the questionnaire suggests MCAS, talk to your healthcare practitioner or allergist about treatment options. There aren’t many in mainstream medicine, but they will help!

If you want more options for treatment, I highly recommend seeking the guidance of a functional medicine professional. In my practice, for example, the top 3 underlying drivers I find in my MCAS patients include excess estrogen (either from estrogen dominance or medications), SIBO/SIFO (small intestinal bacterial/fungal overgrowth), and high histamine diets.

I am accepting new patients if you need guidance, otherwise check out the directory of practitioners at!

Dec 2020 update -
I'm leaving CCM and excited to join the team at Parsley Health!

Megan McElroy
Megan McElroy
Fill out my online form.