What is MCAS?

“My body is constantly having an allergic reaction.”

This is my quick response when people ask what is mast cell activation syndrome (MCAS). In truth, this disease much more complicated than typical allergies.

My mast cells are overactive; they release chemicals, such as histamine, when they aren’t supposed to. These chemicals cause a variety of symptoms such as fatigue, hives, migraine, IBS, bone pain, muscle weakness, vomiting, tachycardia, dizziness, PMDD, and anxiety. Symptoms can range in severity from discomfort to disability. Mast cell reactions can can cause life-threatening complications, including anaphylaxis.

Certain foods and activities can make MCAS worse. For me, these triggers include exercise, fragrances, high histamine foods, hot or cold temperatures, hormone fluctuations, and stress. Everyone is different.

MCAS has been linked to Postural Orthostatic Tachycardia Syndrome (POTS) and Ehlers-Danlos Syndrome (EDS). 

Testing can help diagnose MCAS. However, testing cannot rule out MCAS and false negatives are common. Blood tests for MCAS include serum tryptase, serum chromogranin A, plasma histamine, chilled plasma PGD2, and stat chilled plasma heparin. The 24 hour chilled urine test can measure PGDF2a and n-methylhistamine, but you must keep the container cold AT ALL TIMES. In the United States, PGD2 is now measured as a random, frozen urine test.

MCAS testsTryptase levels are often normal in MCAS patients.

For more information, check out the following resources:

For a longer read, I highly recommend Never Bet Against Occam: Mast Cell Activation Disease and the Modern Epidemics of Chronic Illness and Medical Complexity by Lawrence B. Afrin. Although many MCAS patients have mild symptoms, this book illustrates the complexity of MCAS.