What MCAS is.
The American Academy of Allergy, Asthma & Immunology describes MCAS as a condition in which a person has repeated episodes of the symptoms of anaphylaxis — allergic symptoms such as hives, swelling, low blood pressure, difficulty breathing, and severe diarrhea. The defining feature is the episode: something sets the mast cells off, mediators flood out, and several body systems react at once.
Those episodes cross organ systems, which is part of what makes them recognizable. The AAAAI lists rapid pulse and low blood pressure, itching, hives, swelling and flushing in the skin, wheezing and shortness of breath, and diarrhea, nausea with vomiting, and crampy abdominal pain.
Read that beside a POTS symptom list and you can see why the two get discussed together — a racing pulse, lightheadedness, and gut trouble appear on both. Shared symptoms are a reason for a clinician to look carefully, not evidence that one condition explains the other.
Why the overlap numbers are so slippery.
This is where honesty matters more than a tidy answer. A 2025 study in Frontiers in Neurology looked at exactly this question in a group of 100 young POTS patients and reported that the frequency of MCAS ranged from 2 percent using strict consensus criteria, to 37 percent using more conservative consensus criteria, to 87 percent using loose clinical criteria.
One cohort, one set of patients, and a forty-fold swing in how many of them 'have' MCAS — driven by nothing but the definition applied. That finding should reframe any confident claim you meet about how common MCAS is in POTS.
It is also worth knowing what the main POTS references do not say. The National Institute of Neurological Disorders and Stroke lists conditions associated with POTS — Ehlers-Danlos syndrome type III, myalgic encephalomyelitis/chronic fatigue syndrome, Long COVID, migraine — and mast cell activation syndrome does not appear there. MCAS is a real diagnosis with real criteria and a genuine subject of POTS research. It is not the mainstream explanation for POTS, and anyone presenting it as the answer for most patients has gone past the evidence.
The criteria clinicians actually use.
MCAS has specific consensus criteria, published by Valent and colleagues in the Journal of Allergy and Clinical Immunology. All three must be met: episodic symptoms typical of mast cell activation involving two or more organ systems; a rise in serum tryptase of 20 percent over the person's own baseline plus 2 ng/mL, measured within a four-hour window after the reaction, or a clear rise in another mast cell mediator; and a clear improvement in symptoms with drugs that target mast cell mediators.
The tryptase criterion is the part most often misunderstood. It is not a single number on a single blood draw — it is a comparison against your own baseline, and the timing is unforgiving. The AAAAI notes that total serum mast cell tryptase should be drawn between 30 minutes and two hours after an episode begins, with the baseline obtained many days later. Miss the window and the test cannot answer the question.
The consensus authors are direct about where this goes wrong: a substantial number of patients carrying an MCAS label do not meet the criteria, and the diagnosis should not be applied on the basis of a persistently elevated baseline tryptase alone.
Notice that the third criterion — response to treatment — is part of a process run by a clinician who can prescribe, observe, and reconsider. It is not a reason to assemble a medication or supplement regimen on your own. This page names no protocol, and you should be skeptical of any page that does.
Where this belongs, and when it is an emergency.
Because MCAS episodes are anaphylactic episodes, some of this is time-critical. Sudden trouble breathing, throat or tongue swelling, widespread hives with faintness, or collapse after an exposure needs help immediately — dial 911 rather than wait for a scheduled visit. The AAAAI describes epinephrine as the treatment for acute episodes; if you have been prescribed it, use it as your prescriber instructed.
For the non-urgent evaluation, the referral path is clear, and it is not this office. The AAAAI advises that patients work with their local allergist to coordinate appropriate testing. An allergist or immunologist is the clinician who times the tryptase draw correctly, interprets it against your baseline, weighs the alternatives, and decides whether the criteria are met. If MCAS is a live question for you, ask your primary care provider for that referral.
Gates Brain Health provides complementary functional neurology, and MCAS sits outside what it evaluates or manages. Nothing here substitutes for the allergist who works up your episodes, and any prescribing question — epinephrine included — belongs to the clinician whose name is on the prescription.
Questions worth asking.
POTS appears on the practice's published list of chronic neurological concerns it works with; MCAS does not. Whether an evaluation here adds anything alongside an allergy workup is a conversation, not a foregone conclusion, and it gets decided on a call rather than from a webpage. The treatment page describes what an examination involves.
If you are weighing a call to (775) 507-2000, these are worth raising:
- Should an allergist evaluate my episodes before I look at anything else?
- Given what my workup has already turned up, is an evaluation here worth my time?
- How would care here sit alongside the allergist and cardiologist already involved?
- Is it useful to bring tryptase results or prior POTS testing to a first conversation?
- What happens in the free consultation, and what am I agreeing to afterward?