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The MastAttack 107: The Layperson’s Guide to Understanding Mast Cell Diseases, Part 6

I have answered the 107 questions I have been asked most in the last four years. No jargon. No terminology. Just answers.

12. What do these blood and urine tests look for?

• There are a lot of tests ordered for mast cell disease. How they are interpreted can depend upon a lot of factors. Some of the tests are unreliable, a fact that will be addressed in detail later in this series. (And has been addressed in detail elsewhere on this blog). Please keep in mind when reading this post that I am being VERY general and assumed the test was performed correctly on a correctly stored sample.
• The most common test ordered for mast cell disease is serum tryptase. Tryptase is a molecule that mast cells release. While it has lots of functions in the body, and is especially important in healing wounds and tissue growth, the amount present in your body at a given moment should be low.
• Tryptase is special because mast cells release it in two ways. Firstly, they make and release a little bit steadily. This is not related to activation. Mast cells just normally release a little tryptase as they go about their work. So the idea is that if you have more mast cells than you should, and each of those mast cells releases a little tryptase all the time, that you will have a higher than normal serum tryptase.
• Patients with a clonal mast cell disease, in which they have too many broken mast cells, usually have elevated baseline tryptase. This means tryptase that is elevated at least two times when you are NOT having a big reaction or anaphylaxis.
• Mast cells also store lots of tryptase in their pockets. When the mast cell is activated and it empties out its pockets, lots of tryptase comes out at once. This is why tryptase can be higher after a reaction or anaphylaxis, because mast cells release a bunch at once.
• Patients with mast cell activation syndrome or cutaneous mastocytosis do not always have elevated tryptase even with a big reaction or anaphylaxis.
• Mast cells have huge amounts of histamine stored in their pockets inside their cells. Histamine has lots of functions inside the body and is required for normal body functions. In particular, it is important to our nervous system. Smaller amounts are released as a normal function of the body.
• A lot of histamine is released when mast cells are activated. The idea is that if your mast cells are more activated than they should be that your histamine level will be higher. However, the test recommended for us to consider the histamine level in mast cell patients is not for histamine. It is for n-methylhistamine. This is a molecule that is formed when the body breaks down histamine, which happens very quickly (within minutes of release). n-methylhistamine is more stable, which is why we look at it.
• The test for n-methylhistamine is most reliable when performed in a 24 hour urine sample. This is because the level in urine can fluctuate throughout the day.
• Mast cells make a lot of prostaglandin D2 (abbreviated PGD2). PGD2 is very important for cell communicating. It can carry a message from one cell to another, allowing cells to work together. Unlike histamine and tryptase, mast cells do not keep PGD2 stored in their pockets. They make it only when they need it and then release it.
• PGD2 is released in large amounts when mast cells are activated. However, because it is not stored in the pockets, it is not always elevated right away when you have a big activation event or anaphylaxis. Prostaglandin D2 is broken down quickly. While we do test directly for PGD2 for mast cell disease, we also test for 9a,11-PGF2, a molecule formed when PGD2 breaks down.
• The tests for PGD2 and 9a,11b-PGF2 are most reliable when performed in 24 hour urine samples. This is because the levels in urine can fluctuate throughout the day.
• Heparin is a blood thinning molecule that is stored in pockets inside mast cells. Mast cells are the only cells that release significant amounts of histamine. When the mast cell is activated and it releases histamine, the histamine comes out stuck to heparin. Heparin is broken down very quickly so it is hard to measure accurately.
• The test to assess heparin level actually looks for a molecule called anti-factor Xa that can interact with heparin. This test is performed in serum.
• Chromogranin A is released by mast cells. It is also released by a lot of other cells. The level of this molecule can be affected by many things, including common medications. It is sometimes tested for and considered a sign of mast cell disease if elevated when all other possible reasons can be excluded.
• Chromogranin A levels are most reliable in serum.


For more detailed reading, please visit these posts:

The Provider Primer Series: Management of mast cell mediator symptoms and release

The Provider Primer Series: Mast cell activation syndrome (MCAS)

The Provider Primer Series: Cutaneous Mastocytosis/ Mastocytosis in the Skin

The Provider Primer Series: Diagnosis and natural history of systemic mastocytosis (ISM, SSM, ASM)

The Provider Primer Series: Diagnosis and natural history of systemic mastocytosis (SM-AHD, MCL, MCS)