In short: Mast cell activation syndrome (MCAS) is a condition in which mast cells, a type of immune cell, release chemicals such as histamine too easily, causing recurrent symptoms in more than one part of the body, for example flushing, hives, tummy upset, wheeze or light-headedness. It is diagnosed by matching typical episodes with blood or urine tests taken during symptoms and a response to treatment, after other causes have been ruled out. Experts still disagree about exactly how it should be diagnosed, so careful assessment matters.

This guide, written by a Ready Health prescribing pharmacist, gives a balanced, evidence-based overview of MCAS, how it is investigated and the treatment options a clinician may discuss.

What is mast cell activation syndrome?

Mast cells are found throughout the body, particularly in the skin, gut, airways and around blood vessels. They play a normal role in allergy and defence against infection by releasing chemical messengers (mediators) such as histamine, tryptase, prostaglandins and leukotrienes. In MCAS, these cells are thought to activate inappropriately, without a typical allergic (IgE) cause and without the increased number of mast cells seen in mastocytosis.

What are the symptoms of MCAS?

Symptoms tend to come and go in episodes and usually affect two or more body systems at the same time:

Skin: flushing, itching, hives, swelling
Gut: tummy pain, cramping, nausea, diarrhoea, bloating
Airways: wheeze, throat tightness, nasal congestion
Heart and circulation: fast heartbeat, low blood pressure, light-headedness or fainting
General: fatigue, headache, difficulty concentrating

Reported triggers include heat or sudden temperature changes, exercise, stress, alcohol, certain foods or medicines, fragrances and infections. Many of these symptoms are also common in other conditions, which is why MCAS should not be diagnosed on symptoms alone.

How is MCAS diagnosed?

Widely used expert consensus criteria require all three of the following:

Typical, recurrent episodes of symptoms affecting at least two body systems
Laboratory evidence of mast cell mediator release during an episode, most reliably a rise in blood tryptase taken within about 1 to 4 hours of symptoms, compared with a baseline level taken when well (a rise of at least 20% plus 2 ng/mL)
A clear improvement with treatments that block or stabilise mast cell mediators

Other tests sometimes used include 24-hour urine tests for histamine and prostaglandin breakdown products. These samples must be collected and stored correctly (usually chilled) or results can be falsely normal. Some clinicians and patient groups use broader criteria, and this remains an area of active debate.

Part of the assessment is ruling out other explanations, such as:

Allergies (IgE-mediated food, drug or insect sting allergy)
Systemic mastocytosis or other clonal mast cell disorders, particularly if baseline tryptase is raised
Hereditary alpha-tryptasaemia, a genetic cause of raised baseline tryptase
Thyroid, adrenal and other hormone conditions, coeliac disease, inflammatory bowel disease and some rare tumours

Specialist referral (for example to allergy and clinical immunology, or haematology) is appropriate if a clonal mast cell disorder is suspected, if there has been anaphylaxis, or if symptoms are severe or not responding to first-line treatment.

How is MCAS treated?

Treatment aims to reduce symptoms and is individual. A prescriber will decide what is suitable based on your symptoms, other conditions and medicines. Approaches commonly discussed include:

Identifying and avoiding triggers, ideally with the help of a symptom diary
H1 antihistamines, usually non-sedating types, for flushing, itching and hives
H2 antihistamines, which can help gut symptoms
Leukotriene receptor antagonists for some people with respiratory or skin symptoms
Mast cell stabilisers, such as oral sodium cromoglicate, sometimes used for gut symptoms

Several of these medicines are used outside their UK licence for MCAS, which a prescriber should explain. Treatments are usually started one at a time, at a low dose, so that benefit and side effects can be judged.

Do people with MCAS need an adrenaline auto-injector?

Not everyone does. Adrenaline auto-injectors are recommended for people who have had anaphylaxis or a severe systemic reaction, or who are judged to be at high risk. If you are prescribed them, UK advice is to carry two in-date devices at all times and to have a written emergency plan. If you have symptoms of anaphylaxis, such as difficulty breathing, throat swelling, or feeling faint with a rapid drop in blood pressure, use your auto-injector if you have one and call 999.

Frequently asked questions

Is MCAS the same as mastocytosis?

No. In mastocytosis there is an abnormal increase in mast cells, often with a raised baseline tryptase. In MCAS, mast cell numbers are usually normal but the cells release mediators too readily.

Is MCAS the same as histamine intolerance?

No. Histamine intolerance is thought to relate to reduced breakdown of histamine from food. The two can produce similar symptoms, which is one reason careful assessment is needed.

Can a normal tryptase test rule out MCAS?

Not on its own, particularly if the sample was not taken at the right time after symptoms. Equally, abnormal test results need to be interpreted alongside your symptoms.

Is MCAS linked to other conditions?

MCAS is often reported alongside conditions such as hypermobility syndromes and postural tachycardia syndrome (PoTS), but research into these links is still developing.

Key takeaways

MCAS causes recurrent episodes of symptoms across two or more body systems.
Diagnosis depends on symptoms, correctly timed laboratory tests and response to treatment.
Other conditions, including allergy and mastocytosis, must be considered.
Treatment is individual and usually starts with antihistamines and trigger management.
Anyone at risk of anaphylaxis should have an emergency plan.

Sources and further reading

Last reviewed: October 2026 by Shamir Patel, Independent Prescribing Pharmacist, Ready Health. This article is general information and does not replace advice from your own clinician.

If you think you may have MCAS or a histamine-related condition, you can find out about our MCAS consultation (online or in Standish, Wigan) or book a private GP appointment. Ready Health does not have an in-house immunologist; if specialist investigation is needed, we will discuss referral options with you.