Mast Cell Activation Syndrome (MCAS)
Mast cells are immune cells that release histamine and other chemicals during allergic reactions. In MCAS, they release these chemicals too easily, causing repeated episodes of flushing, hives, swelling, wheeze, low blood pressure or fainting, and tummy cramps or diarrhoea, in two or more parts of the body at once. True MCAS is uncommon. Most people with long-term, varied symptoms don’t meet the criteria, so other causes need checking. An immunologist or allergist usually makes the diagnosis.
How MCAS is diagnosed. All three are needed: (1) episodes of typical symptoms in at least 2 body systems at the same time; (2) blood tryptase rises during an episode to more than 20% above your usual level plus 2 (for example, from a usual level of 5 to 8 or more), compared with a level taken 24 hours or more after symptoms settle; and (3) symptoms improve with mast cell medicines. POTS and Ehlers-Danlos syndrome are not part of the criteria.
Treatment options
Ratings show how strong the research is.
- Strong evidenceAdrenaline autoinjector, if you’ve had anaphylaxisEpiPen or Anapen, with an ASCIA Action Plan and training.
- Moderate evidenceNon-sedating antihistaminesSuch as cetirizine, fexofenadine or desloratadine, taken every day. The first treatment to try. Your specialist may advise a higher dose.
- Limited evidenceFamotidineAn acid-lowering medicine that also blocks histamine. Added for flushing or gut symptoms.
- Limited evidenceMontelukastMay help wheeze or flushing. Stop it and tell your doctor if you notice mood or sleep changes.
- Limited evidenceMast cell stabilisersSodium cromoglicate taken by mouth before meals, or ketotifen. Mainly for gut symptoms, under specialist guidance.
- Limited evidenceOmalizumabA specialist injection for severe, repeated anaphylaxis despite other treatment.
- Not recommendedStrict low-histamine or elimination dietsNo proven benefit for MCAS, and they risk poor nutrition. If you change your diet, see a dietitian with allergy experience.
- Not recommendedBlood or urine histamine tests, and IgG food testsNot reliable for diagnosing MCAS, so ASCIA advises against them.
What you can do
- Keep a diary of episodes and possible triggers, such as heat, exercise, alcohol, stress, infections, insect stings, or medicines such as anti-inflammatories and opioids.
- Ask your GP for a tryptase request form to keep at home, so you can have blood taken 30 minutes to 2 hours after an episode starts.
- Wear medical ID and carry your adrenaline autoinjector at all times if prescribed.
- Tell doctors, dentists and anaesthetists before procedures.
See your GP if
- You have repeated episodes of flushing, hives or swelling with faintness, wheeze or tummy cramps
- You’ve been told you have MCAS without a tryptase test during an episode
- Treatment isn’t working, or you’re cutting out more and more foods
- Your symptoms are ongoing rather than in episodes: other causes are more likely and need checking
Call 000 and use your adrenaline autoinjector for any sign of anaphylaxis: trouble breathing, wheeze or persistent cough, swollen tongue or tight throat, hoarse voice, dizziness or collapse. Lie flat (or sit with legs out if breathing is hard), and don’t stand or walk.
Evidence and more information
- ASCIA: Mast cell activation disorders and syndrome, position paper (2025)
- ASCIA: Testing for mast cell activation disorders
- ASCIA: First aid for anaphylaxis
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