"We don't really 'believe in' MCAS here."— what you've probably been told
MCAS
MCAS is recognised by the British Society for Allergy and Clinical Immunology (BSACI) and is diagnosed against the international consensus criteria (Valent et al., 2012, updated 2019): (1) typical multi-system mast cell mediator symptoms, (2) objective evidence of mast cell 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).
The wait: Patients are routinely sent in circles between specialties for years before tryptase is checked.
MCAS often presents with multi-system symptoms (flushing, GI, neurological, dermatological, cardiovascular) that don't fit a single specialty. GPs frequently send patients in circles between services rather than testing tryptase. The single most common reason an MCAS diagnosis fails is that nobody captures acute tryptase within the 1–4 hour window after a flare — which requires a standing phlebotomy form held at the practice in advance.
- 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
Is MCAS recognised in the UK?
Yes. The BSACI publishes guidance, NHS adult immunology and allergy services in major teaching hospitals diagnose it, and the international consensus criteria (Valent et al., 2012/2019) are the recognised diagnostic standard.
I think I have CIRS. Will this letter work for me?
CIRS (Chronic Inflammatory Response Syndrome) isn't recognised by NICE or the NHS, so a letter demanding 'treat my CIRS' wouldn't get far. But MCAS is recognised by BSACI, and many CIRS symptoms overlap with mast-cell, post-viral and autonomic presentations the NHS does investigate. Our letter leans on those recognised pathways rather than the CIRS label. See the full explainer at /guides/cirs-uk-nhs-recognition.
What's usually said in the room
"We don't really 'believe in' MCAS here."
What the guideline actually says
MCAS often presents with multi-system symptoms (flushing, GI, neurological, dermatological, cardiovascular) that don't fit a single specialty. GPs frequently send patients in circles between services rather than testing tryptase. The single most common reason an MCAS diagnosis fails is that nobody captures acute tryptase within the 1–4 hour window after a flare — which requires a standing phlebotomy form held at the practice in advance.
Source: British Society for Allergy and Clinical Immunology — mast cell disorders (BSACI guidance)
Outcomes from people with MCAS
One short email each Sunday — anonymised stories from people who got their GP to take them seriously.