Mast Cell Activation Syndrome: Empiric Treatment Trial Versus Extensive Mediator Workup
A court stenographer has spent eight months documenting her own flushing and tachycardia and wants to start mast-cell-directed therapy today. The disagreement is whether her one acute lab value actually says what she thinks it says.
K.B., a 31-year-old courtroom stenographer, has spent the last eight months tracking her own symptoms in a spreadsheet she brought to this visit unprompted: flushing, a racing heart, cramping abdominal pain, occasional lightheadedness, appearing sometimes with heat or a stressful deposition and sometimes with nothing she can name at all. Four emergency-department visits and two urgent-care trips have documented tachycardia and flushing but never a blood pressure low enough, nor a throat tight enough, to call any episode anaphylaxis. A tryptase drawn during her worst documented episode came back at 6.8 ng/mL — inside the normal range, though nobody thought to also draw a symptom-free baseline for comparison, so whether that number represents a real rise or simply where her tryptase always sits is genuinely unknown. She arrives having read extensively about mast cell activation syndrome and asks directly for antihistamines, cromolyn, and an H2 blocker today, visibly bracing for another referral to yet another specialist instead.
The 2019 international consensus criteria for MCAS, the framework most allergists now use, require more than a plausible symptom pattern: a documented rise in tryptase or another mast cell mediator of at least 20 percent plus 2 nanograms per milliliter above a patient's own symptom-free baseline, drawn within four hours of an episode, together with a clinical response to mediator-targeted therapy. Nothing in K.B.'s chart meets that bar yet — not because her symptoms aren't real, but because no comparison baseline has ever been drawn and her one acute value sits within normal limits regardless. Valent and colleagues, and separately more recent critiques of the diagnosis's rising popularity, both make the same point from different directions: a broad empiric trial of mast-cell-directed therapy, started before that mediator documentation exists, risks treating a label she found herself rather than confirming or ruling out the diagnosis it's supposed to represent — and risks missing a different, equally real driver that a mast-cell framework would never surface.
At the bedside
She has spent eight months documenting this herself because nobody else was. Four emergency-room visits, one urgent-care trip, and she still doesn't have a plan that goes further than ‘follow up as needed.’ I want to start her on a standard antihistamine and H2-blocker regimen today. It's low-risk, it's exactly what most allergy practices try first regardless of formal criteria, and continuing to make her wait for a perfect diagnostic label while she's missing depositions isn't a neutral choice — it has its own cost.
I don't doubt she's suffering. I doubt that starting broad mast-cell-directed therapy without ever drawing a comparison baseline gets us anywhere useful. The 2019 consensus criteria exist because ‘flushing plus tachycardia plus a normal tryptase’ describes a lot of conditions that aren't MCAS — anxiety disorders, postural tachycardia syndrome, even early carcinoid physiology can all look like this from across the room. If we start her on cromolyn today and she feels better in three weeks, we will have taught ourselves nothing about which of those it actually was, and she'll have a diagnosis on her problem list that may not be the right one.
I take the low-risk argument seriously. I don't think it answers what she actually has.
I don't think either of you actually disagrees about what she should be tested for — you disagree about whether treating and testing have to happen in sequence, and I don't think they do. Give her the kit to capture tryptase, N-methylhistamine, and PGD2 metabolites at her very next episode, with a symptom-free baseline drawn this week for comparison, and start the antihistamine and H2 blocker today regardless. If the mediator data comes back positive, we've confirmed MCAS on schedule. If it comes back flat while she's already improved on medication, that itself is useful information — it points away from mast-cell mechanism and toward something else, without having cost her another month of untreated symptoms to learn it.
Agreed: start cetirizine and famotidine today; symptom-free baseline tryptase drawn this visit; she leaves with a home kit and clear instructions to get tryptase, N-methylhistamine, and PGD2 metabolites drawn within four hours of her next episode.