"We don't really 'believe in' MCAS here.", what you've probably been told
MCAS
No dedicated NICE guideline exists for MCAS, and there is no BSACI MCAS guideline. Your letter is grounded in the NHS Constitution for England and GMC Good Medical Practice, asking your GP to arrange the investigations an NHS immunology or allergy service would need to see. Once you reach specialist immunology, the international Valent consensus criteria (2012, updated 2019) are the framework a specialist will use, requiring a rise in serum tryptase of at least 20% above baseline plus 2 ng/mL measured within 1 to 4 hours of a symptomatic episode. This page describes the specialist framework for your context, the letter itself does not cite Valent as if it were NICE guidance.
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 to 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 immunology services will accept, 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
- Referral to NHS adult immunology or allergy. Individual services set their own referral criteria and some publish an explicit refusal to see suspected MCAS, so a referral should name the investigations above rather than request a mast cell service
- A trial of H1 and H2 antihistamines (for example fexofenadine plus famotidine, at a dose your GP decides) while awaiting specialist review, as both diagnostic and therapeutic per the consensus criteria
Ready to act on what you just read?
Get a formal MCAS letter to your GP, drafted, cited, and sent in minutes.
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 the guidance 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 to 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 immunology services will accept, the realistic ask is symptom documentation, tryptase capture and immunology review, not a guaranteed MCAS label.
Source: NHS Constitution for England + GMC Good Medical Practice (duty to refer)
What to do next
You have just read what the guidance says. The gap most people hit next is getting that in writing, on their record, in front of their GP.
Your GP has not acted on it
We draft a formal letter to your NHS GP citing the applicable NHS guidance, built from a 3 minute assessment in your own words. £39. Free redraft if anything reads wrong.
You send the letter, not us. General information, not medical advice.
Benefits and rights for MCAS
Outcomes from people with MCAS
One short email each Sunday, anonymised stories from people who put their case to their GP in writing.