Diagnosing Mast Cell Activation Syndrome (MCAS)


What is mast cell activation syndrome?

Mast cell activation syndrome (MCAS) is a condition that impacts the body’s mast cells. Mast cells grow in the skin, airway, gastrointestinal tract, and bone marrow. They are blood cells that play a part in the immune system, and are also involved in allergic reactions. They release chemicals called “mediators” when the body encounters an allergen.

In mast cell activation syndrome, the mast cells release too many mediators when exposed to allergens or other substances, resulting in severe allergy symptoms. This causes problems with the heart, skin, neurologic system, and gastrointestinal tract. Although there is no cure for MCAS, treatment typically involves the avoidance of triggers and medication for symptom management.

Diagnostic process

MCAS can present symptoms of other conditions; therefore, it is difficult to diagnose. A conclusive diagnosis requires ruling out these conditions. Diagnosing MCAS includes obtaining a medical history, performing a physical examination, conducting tests, and completing medication trials.

Medical history

A health care professional will review a medical history to rule out other possible conditions. Additionally, symptoms will be evaluated to ensure they fit the criteria of MCAS. The guidelines for diagnosing MCAS is debatable; however, a physician typically evaluates the following:

  • A decrease in symptoms when taking antihistamine medications that block mediator release
  • Severe or repeated symptoms affecting at least two organs
Physical examination

A physical examination determines the presence and severity of chronic allergy symptoms before and after medication.


Although there are no definitive tests for MCAS, certain testing can help with diagnosing, which includes the following:

  • A blood test detects higher levels of histamine, tryptase, or prostaglandins during a flare of MCAS.
  • A urine test examines higher levels of N-methylhistamine (NMH) during an episode of MCAS.
  • A bone marrow biopsy reviews certain patterns and markers in bone marrow that can be indicative of mast cell activation disorders.
Medication trial

In order to diagnose MCAS, a health care professional will observe any symptom improvement that occurs when treated with certain medications that are prescribed for MCAS. If symptoms improve with these medications, a diagnosis of MCAS is likely.

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