Diagnosing MCAS requires a systematic, stepwise approach, which means that it can take a considerable amount of time for individuals to receive a formal diagnosis of MCAS.
Key steps in diagnosis MCAS include recognising clinical symptoms, investigating response to treatment, undertaking mediator tests and ruling out other diagnoses. Sometimes, not all of these steps are, or can be, taken. In these cases, people are typically diagnosed with ‘suspected MCAS’. Expand the sections below to learn more about these steps in MCAS diagnosis.
The first step is recognising the presence of typical clinical symptoms across multiple body systems.
MCAS symptoms are often episodic or wax and wane (meaning that they go and come back) and their severity can fluctuate over time.
Symptoms can include:
Another step in the diagnosis of MCAS is often referred to as a ‘test of treatment’. It is underpinned by the theory that, if a person responds to MCAS treatment, this is evidence to suggest that the individual has MCAS.
Not everyone with MCAS will respond to the same medicines, so finding one that works can be a process of trial and error.
Please seek medical support:
Some of the medicines mentioned in this list are available over the counter from the pharmacist, but many require a prescription from your doctor. Because some medications can also be triggers for some people with MCAS, we recommend you seek medical guidance either from a pharmacist or other medical health professional when using any of these medicines to manage the symptoms of MCAS.
Common medications that can be used for MCAS include:
Ideally, the previous steps are combined with a biochemical assessment to show evidence of elevated mediator release from mast cells.
Finally, it is important to discount other potential diagnoses, or determine whether an individual is suffering multiple conditions (with or without MCAS).
Common comorbidities include connective tissue disorders like Ehlers Danlos Syndrome (EDS), and Marfans. As well as Postural Orthostatic Tachycardia Syndrome (PoTS), and Type 2 Diabetes. Along with Anaphylaxis, Autoimmune diseases, Autoinflammatory conditions, Vasculitis or Addison’s Disease.
For those who suffer from Anaphylaxis we recommend understanding how to prevent and treat it.
This stepwise process is reflected in the published diagnostic criteria for the condition (Molderings et al., 2011; Valent et al., 2019) and in the diagnostic algorithm proposed by Giannetti et al., 2021). If a person fulfils each of these four sets of criteria, a confident MCAS diagnosis can be provided. Frequently this process requires a ‘trial and error’ approach, and people may have to wait a long time for a definitive diagnosis. The uncertainty of this situation can be very difficult for people who suspect that they have MCAS but have not had their condition diagnosed. If you have been affected by this, please do reach out for further information and support from the charity.
If you are a medical professional and would like to access more information related to MCAS diagnosis, please see the Diagnostic Support section of our Resources for Medical Professionals page, as well as the MCAS Bibliography.
Weiler CR. Mast Cell Activation Syndrome: Tools for Diagnosis and Differential Diagnosis. J Allergy Clin Immunol Pract. 2020;8(2):498-506.
The following papers are key resources for diagnostic tests.
Butterfield J, Weiler CR. The utility of measuring urinary metabolites of mast cell mediators in systemic mastocytosis and mast cell activation syndrome. The Journal of Allergy and Clinical Immunology: In Practice. 2020 Sep 1;8(8):2533-41.26.05.2021
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