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, IBS, bone pain, muscle weakness, nausea, tachycardia, dizziness, PMDD, and anxiety. Symptom severity can range from discomfort to disability. Severe reactions can even cause life-threatening 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 false negatives are common. Testing cannot rule out MCAS. 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 is best for measuring PGD2 and n-methylhistamine, but you must keep the container cold AT ALL TIMES.

Blood tests for mast cell activation (MCAS) include serum tryptase, serum chromogranin A, plasma histamine, chilled plasma PGD2, and stat chilled plasma heparin. The 24 hour chilled urine test is best for measuring PGD2 and n-methylhistamine.Tryptase 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.