So You Think You Have MCAS… What Next?!

by | Nov 15, 2017 | All About MCAS |

I’ll never forget the day I discovered Dr. Afrin’s book “Never Bet Against Occam: Mast Cell Activation Disease and the Modern Epidemics of Chronic Illness and Medical Complexity.” In a short time frame, I experienced 17 years-worth of suppressed emotions over finding an explanation for decades of unexplained symptoms.

This extreme sense of hope had happened a few times in the past, so I was still somewhat guarded. However, no other prior misdiagnosis had been able to fully explain the full realm of what I had been experiencing. I warned myself not to get my hopes up too much, but nonetheless I rather joyfully skipped in to my doctor’s office with a list of the recommended tests for MCAS.

Fortunately, my functional medicine doctor was willing to test me for MCAS even though she’d never heard of it. And fortunately, I had tests come back astoundingly positive during the first round of testing, which is not always the case. The true clinical diagnosis of MCAS is generally tricky to nail down, at best.

With a disease that is as obscure as MCAS in mainstream medicine, the diagnosis itself can be a somewhat painful and lengthy process. Below are some tips to help you navigate the process from your initial suspicion of MCAS to diagnosis.

Read up on the disease.

Check out the two different suggested guidelines for clinical diagnosis on the “Diagnosis” tab of “What is MCAS?” Keep in mind that there is not one current globally accepted version as of 2017. See the “Reading List” and “Resources” section for more information.

Find an open-minded doctor, and not necessarily a “specialist in MCAS” for initial testing.

The (few) specialists in the U.S. have long (often several year) wait lists. Your best bet is to find a clinician who takes a thorough history, believes you, and is willing to learn about MCAS with you and test you for it. If your doctor is unfamiliar with the disease, print out an article on MCAS to bring to your appointment that outlines the disease and suggestions for clinical testing. You can find some useful articles here that have algorithms for clinical diagnosis:

If you don’t have any luck with your usual medical team, pursue new options.

In my personal experience, functional medicine doctors and naturopaths may be more open or knowledgeable about MCAS than my mainstream primary care options. The “Mast Movement” Facebook group has a list of doctors listed by geographical area who are knowledgeable about MCAS.

Prior to initial laboratory testing, be well-versed in the special considerations for the lab work.

The 24-hour urine tests need to be continuously chilled, for example, so it’s important that you keep them in your fridge, put them on ice for transport, and make sure the lab gets them into a chilled environment right away and for future transport. Many medications can alter the test results, so it’s also important to make the necessary changes for a short period prior to testing (most say 5 days) as long as you are medically stable enough to do so. The chart *at the end of this post* describes the more commonly used MCAS lab tests and the reference values and special considerations for each.

Find social support.

Get connected to a support group or join our mentoring program. There are also many facebook groups out there that can help you answer specific questions that may come up in the process.

If at first you don’t succeed…

If your lab tests come back negative, keep in mind that it may take another round of testing (which is generally more likely to be positive during a flare). It’s also important that your doctor considers other conditions which could mimic MCAS.

Consider secondary testing.

Discuss the consideration of having other tissue tested with your doctor. Staining of tissue from a previous biopsy of a specific area (like the gastrointestinal tract) can help rule in the diagnosis of MCAS (though it does not rule it out if it’s normal). CD2, CD25, and CD-117 staining are not routinely ordered and need to be specifically requested.

Bone Marrow Biopsy?

Depending on your findings, a bone marrow biopsy may be warranted, though this is not common with MCAS patients. Generally, bone marrow biopsies are performed if your tryptase level is greater than 20 or if you experience frequent fainting episodes or other red flags indicative of Mastocytosis.

As you move forward to treatment, be patient!

The diagnosis is part of the battle, but there is often a lengthier trial and error process in order to find the right medication and/or supplement combination that can help you find a less symptomatic baseline.

Visit our next blog post, Checklist for the Newly Diagnosed for more guidance!



Common Chemical Mediators Tested in Patients with Suspected MCAD

Mast Cell Mediator Interpretation of Value Clinical Notes (2017) Reference for Lab Values
24-hour Urine N-methyhistamine (NMHIN) Normal:

Age 0-5: 120-510 mcg/g creatinine

Age 6-16: 70-330 mcg/g creatinine

Age >16: 30-200 mcg/g creatinine

HIGH levels common with MCAD; Keep sample chilled continuously; diet can influence histamine level by ~30%; DAO supplementation and anti-histamine medications can affect this test result Mayo Clinic
24-hour Urine Prostaglandin D2



100-280 ng (both Mayo Clinic and Inter Science Institute Laboratory)

HIGH levels common with MCAD; Keep sample chilled continuously; aspirin and other NSAIDs can affect this test result; urine more accurate than blood test Mayo Clinic/ Inter Science Institute
24-hour Urine Prostaglandin 9a,11b-F2


< 5,205 pg/mg creatinine (Mayo)

375-800 ng (Inter Science Institute)

HIGH levels common with MCAD; PGF2a is (more stable) metabolite of PGD2 (PGF2a believed to be superior for MCAS detection); urine more accurate than blood test Mayo Clinic/ Inter Science Institute
Tryptase (serum) Median normal values:

Age <6 months: 6 ng/ml

Children & Adults: 3-4ng/ml


High >11.5 ng/ml

Criterion for consideration of SM: >20 ng/ml

HIGH levels common with MCAD; Baseline testing at least 24 hours after acute episode; acute measurement within 15 mins-4 hours of episode Mayo Clinic
24 hour Urine Leukotriene E4



< or = to 104 pg/mg creatinine

HIGH levels common with MCAD; Keep sample chilled continuously; Zileuton/Zyflo medication may affect this test result Mayo Clinic
Serotonin (serum) Normal:

< or = to 230 ng/ml


Other sources* consider normal to be:

101-283 ng/ml

High levels may suggest carcinoid tumors; LOW levels associated with MCAD**; MAO inhibitors, lithium and methyldopa, morphine & other medications can affect this test result Mayo Clinic
stat chilled Plasma Heparin (PH) via Chromogenic Anti-Factor Assay

“Anti-XA Assay”

Upper reference value of 0.02 to 0.05 anti-Xa IU/ml, depending on lab HIGH levels common with MCAD; Specific/sensitive mast cell marker, but metabolizes quickly even when refrigerated so has to be centrifuged within 30 mins. Vysniauskaite et al, 2015
Serum Chromogranin A Normal:

< 93 ng/ml (all ages)

HIGH levels common with MCAD; high levels may also indicate heart conditions, kidney problems or neuroendocrine tumors; proton pump inhibitors can affect this test result Mayo Clinic


This content is Copyright © Mast Cells United and is not intended to diagnose or treat anyone. Always consult your medical professional for any health guidance or advice.

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