How is MCAS diagnosed?
We know how important getting a confirmed diagnosis of MCAS can be. Published guidance recommends four diagnostic stages that should be completed so that your doctor feels comfortable confirming a full diagnosis of MCAS. Getting this diagnosis can be a complex and lengthy process taking many years, particularly due to the challenges involved with testing for mast cell mediators.
Stage 1
MCAS is considered a multi-systemic condition, which means that two or more body systems are affected at the same time during a reaction. Because of the complex pattern of symptoms, no two people are likely to experience MCAS in exactly the same way. Symptoms can also vary over time, often waxing and waning.
It is common for someone with MCAS to have mild and controllable symptoms for a long time, but a stressful event like an infection, an operation or an illness – for example COVID-19 – can cause a dramatic increase in symptom severity or frequency. Again this can make recognising the symptoms of MCAS quite challenging.
Stage 2
This stage ascertains ifMCAS symptoms are improving with certain medications. This may bereferred to as a medication trial - if a person’s symptoms get better after taking a known MCAS medication, then a diagnosis is more likely.
You can find out more about MCAS medications here.
Stage 3
There are tests that can be used to identify whether higher than normal levels of chemical mediators are present in the body. These mediators can be tested for in the blood or in the urine.
Due to the allergic-type symptoms of MCAS, testing for IgE-mediated allergies is often one of the first tests to be performed by doctors, usually as a skin prick test or by taking a blood sample for RAST testing. In idiopathic MCAS these are usually found to be inconclusive or negative.
A blood test to evaluate tryptase levels is often performed, particularly as it is a strong marker for mastocytosis and for HATs, but it is considered less specific for MCAS as many people affected by MCAS do not have high baseline tryptase levels. This means that their normal, everyday tryptase levels are not high. They may however be higher during a symptom flare or during anaphylaxis, but it can be challenging to obtain blood samples at this precise time for testing.
Other mast-cell mediators can be tested for with a 24-hour urine collection. These include methylhistamine and prostaglandins. Elevated results of these can be suggestive of MCAS but having normal levels doesn’t rule out MCAS and repeated testing may be needed.
It is often better to take these tests during a flare, when symptoms may be particularly severe. In addition, some tests may need to be performed in the absence of particular medications, which will need to be carefully discussed with a health professional. However, we know that for some people it can be difficult to choose to stop treatment, particularly when it is effective, and this is an important consideration when seeking a fully ‘biochemically-confirmed’ diagnosis.
Biopsies can also be taken from the gut, bladder or skin, where the presence of high or abnormal mast cell numbers can be supportive of a diagnosis of MCAS or mastocytosis.
Unfortunately, there are currently no genetic tests identified for diagnosing MCAS – but research is underway to change this.
Stage 4
Eliminating any other possible causes of symptoms is an important stage of the diagnostic process as there are many conditions that overlap with MCAS such as ME/CFS, Lyme, andasthma, for example.
Due to the wide-ranging symptoms associated with MCAS there can be an overlap with the symptoms experienced with a number of other conditions. During any diagnostic process, it is important to ensure that other potential diagnoses are not missed. In addition, more than one condition can exist at the same time. This may mean additional investigations such as endoscopy, colonoscopy, imaging and biopsies are required.
As MCAS may not have been the first thing that your doctor suspected, you may have undergone some of these tests already. We appreciate that this can be frustrating, and some tests can be unpleasant, but it can help to remember that for a confirmed diagnosis of MCAS it is important that other potential diagnoses have been excluded.
Other conditions that are important to rule out include heart attacks, hepatitis, cancer, epilepsy, oesinophilic oesophagitis, Addison’s disease and diabetes.
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