"We don't really 'believe in' MCAS here.", what you've probably been told
MCAS
MCAS is diagnosed against the international consensus criteria (Valent et al., 2012, updated 2019): (1) recurrent, severe multi-system symptoms typical of mast cell mediator release (the consensus framing assumes anaphylaxis-level episodes, not background symptoms alone), (2) objective evidence of mediator release, most commonly a rise in serum tryptase of at least 20% above baseline plus 2 ng/mL, measured within 1–4 hours of a symptomatic episode, and (3) response to mast-cell-directed treatment (H1/H2 antihistamines, mast cell stabilisers). NHS adult immunology and allergy services in major teaching hospitals do investigate and diagnose it where those criteria are met. The looser 'consensus-2' (Afrin) criteria used in some patient communities are not the standard NHS immunology will accept.
The wait: Patients are routinely sent in circles between specialties for years before tryptase is checked.
What GPs miss
MCAS can present with multi-system symptoms (flushing, GI, neurological, dermatological, cardiovascular) that don't fit a single specialty, and GPs often send patients between services rather than capturing tryptase. The most common reason a diagnosis fails is that nobody captures acute serum tryptase within the 1–4 hour window after a flare, which requires a standing phlebotomy form held at the practice in advance. Where symptoms don't reach the severity threshold the Valent criteria assume, the realistic ask is symptom documentation, tryptase capture and immunology review, not a guaranteed MCAS label.
The letter asks for
- Baseline serum tryptase, total IgE, FBC and vitamin D
- A standing phlebotomy form for acute serum tryptase, to be drawn within 1–4 hours of a symptomatic episode (the key diagnostic test under the Valent consensus criteria)
- Referral to NHS adult immunology or allergy, citing the international consensus criteria (Valent et al., 2012/2019) for MCAS
- Where applicable, a Right to Choose referral to an NHS-contracted mast cell service with a shorter waiting list
- A trial of H1 and H2 antihistamines (e.g. fexofenadine 180mg twice daily plus famotidine 20mg twice daily) while awaiting specialist review, as both diagnostic and therapeutic per the consensus criteria
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Frequently asked questions
Is MCAS recognised in the UK?
Yes, narrowly. NHS adult immunology and allergy services in major teaching hospitals diagnose it under the Valent international consensus criteria (2012, updated 2019). Individual NHS services publish their own referral criteria, e.g. Leeds Teaching Hospitals Allergy & Clinical Immunology, and most will only accept referrals where symptoms reach the severity the consensus framing assumes. There is no BSACI MCAS guideline.
I think I have CIRS. Will this letter work for me?
CIRS (Chronic Inflammatory Response Syndrome) isn't recognised by NHS guidelines or the NHS, so a letter demanding 'treat my CIRS' wouldn't get far. But many CIRS symptoms overlap with mast-cell, post-viral and autonomic presentations the NHS does investigate. MCAS (where Valent criteria are met), Long COVID under NHS guideline NG188, ME/CFS under NG206. Our letter leans on those recognised pathways rather than the CIRS label. See the full explainer at /guides/cirs-uk-nhs-recognition.
What you hear vs. what Valent 2012/2019 says
What's usually said in the room
"We don't really 'believe in' MCAS here."
What the guideline actually says
MCAS can present with multi-system symptoms (flushing, GI, neurological, dermatological, cardiovascular) that don't fit a single specialty, and GPs often send patients between services rather than capturing tryptase. The most common reason a diagnosis fails is that nobody captures acute serum tryptase within the 1–4 hour window after a flare, which requires a standing phlebotomy form held at the practice in advance. Where symptoms don't reach the severity threshold the Valent criteria assume, the realistic ask is symptom documentation, tryptase capture and immunology review, not a guaranteed MCAS label.
Benefits and rights for MCAS
Outcomes from people with MCAS
One short email each Sunday, anonymised stories from people who got their GP to take them seriously.