Doctor, I Think I Am Suffering from MCAS: Differential Diagn
MCAS is a well-defined rare condition that occurs most frequently in patients with IgE-dependent allergies and/or SM, but can rarely also occur on the basis of other conditions. In a few cases, no underlying disease is found leading to the diagnosis of “idiopathic MCAS.” Diagnostic MCAS consensus criteria have been established and should be employed in each case.

A key diagnostic marker is the event-related increase in MC tryptase over the individual's baseline, measured in the symptom-free interval. When the tryptase elevation exceeds a certain threshold (20% from baseline plus 2 ng/mL) the diagnosis of MCAS is very likely.

In addition, a number of conditions and disorders can mimic symptoms of MCA or MCAS. These conditions have to be considered in each case, especially when MCAS criteria are not fulfilled. This is important as some of these differential diagnoses are serious or even life-threatening and thus need to be identified instead of misdiagnosing the case as MCAS.

Researchers recommend the following questions to be answered by patients who were informed to (probably or maybe or definitively) suffer from MCAS:
1. Did my symptoms repeatedly occur in the form of severe attacks requiring immediate medical intervention and/or hospitalization?
2. Did my symptoms lead to an anaphylactic shock requiring hospitalization?
3. Did my doctor(s) measure serum tryptase levels before, during, and after my attacks?
4. Did my doctor(s) tell me that my tryptase levels increased during my attacks?
5. Did my symptoms improve with continuous treatment with antihistamines?
6. Did the frequency of severe attacks decrease since I took steroids or antihistamines?
7. Did my doctor(s) diagnose an IgE-dependent allergy?
8. Did my attacks resolve or decrease in number after I started with omalizumab?

When most of these questions are answered with “yes,” the likelihood of MCAS is rather high. When most of these questions are answered with “no,” an MCAS can essentially be ruled out or is very unlikely.