MCAS: A tale of two consensuses
This post is a little bit tongue-in-cheek. But only a little bit!
In the medical world, when an illness is new enough that there aren't official clinical guidelines for it yet, a group of experts will often come together to write a 'consensus statement'. This consensus becomes a de facto guideline that doctors use to guide them in diagnosing, treating and managing the condition.
In the world of MCAS there are two consensuses, which say very different things. I find this hilarious. The doctors involved either didn't know what the word 'consensus' means, or they didn't know that a lot of their colleagues disagreed with them, or they did know but hoped no-one would notice. In any case, it's hilarious.
Consensus 1 (the bad one)
Consensus 1 requires three things for a diagnosis of MCAS:
1) Typical MCAS symptoms
2) A blood test showing an increase in serum tryptase, or an increase in some other chemical derived from mast cells
3) Symptoms improve with drugs that target mast cells
The problem with requirement (2) is that while a positive test for tryptase definitely indicates MCAS, a negative test doesn't really mean anything. In medical parlance, this test is specific but not sensitive. Requirement (2) makes sure you won't give out an MCAS diagnosis to people who don't have it, but it also ensures that you'll miss out on diagnosing a lot of people who do have it.
Why did they do it this way? My best guess is that the doctors who created Consensus 1 weren't really focused on helping patients. Their main goal was to demonstrate to their medical colleagues that they are Very Serious Doctors who do Serious Clinical Tests. They don't just hand out diagnoses willy-nilly to patients who whine and complain about their life-altering symptoms! They are far too Clever and Serious for that!
Consensus 2 (the good one)
Consensus 2 basically says that if someone has the symptoms of MCAS, and they improve significantly when they take drugs that target mast cells, then it's almost certainly MCAS. No need to do blood tests that don't even work very well anyway.
The main idea behind Consensus 2 seems to be that it would be good to... provide medical care to patients? If possible? Crazy, I know.
What this means for patients
If you see an MCAS specialist, you might want to check that they adhere to Consensus 2, not Consensus 1. I would think that most specialists in the worlds of ME/CFS, long Covid, fibromyalgia and POTS would adhere to Consensus 2, but it can't hurt to check.
Further reading
• Consensus 1 (the bad one): Valent P et al. Definitions, criteria and global classification of mast cell disorders with special reference to mast cell activation syndromes: a consensus proposal. Int Arch Allergy Immunol. 2012;157(3):215-25. doi: 10.1159/000328760. Epub 2011 Oct 27. PMID: 22041891; PMCID: PMC3224511.
• Consensus 2 (the good one): Afrin LB et al. Diagnosis of mast cell activation syndrome: a global "consensus-2". Diagnosis (Berl). 2020 Apr 22;8(2):137-152. doi: 10.1515/dx-2020-0005. PMID: 32324159.
Thank you for getting the word out about this. Conflicting "consensus" criteria has made life difficult for people with ME as well.
I think when patients realize doctors use labels that may have different/conflicting criteria, they can be empowered to ask which criteria a doctor uses....
Dr Afrin definitely looked beyond the tryptase yet he says the immune system is mistaken.
Which implies there is nothing there to be tested for.
Which, in effect, tells researchers not to bother looking for an unknown environmental agent.