Allergy and intolerance
Tryptase
A protein from mast cells; the resting value says something about the number of mast cells, and it rises shortly after a severe allergic reaction.
Everything about this testWhat it measures
Tryptase is a protein stored in mast cells, a type of white blood cell involved in allergic reactions. When mast cells are activated, they release histamine and tryptase into the blood. The value measured at rest, the basal tryptase, is mainly related to the number of mast cells in the body.
- A sign of mastocytosis, a condition in which there are too many mast cells in the body
- Confirmation afterwards of a severe allergic reaction (anaphylaxis), if the blood was taken shortly after the reaction
- A hereditary raised resting value due to hereditary alpha tryptasemia, in which someone has extra copies of the tryptase gene
- Rarely: a raised value in a bone marrow disease
High
A slightly raised resting value often has a harmless explanation. A known cause is hereditary alpha tryptasemia: people with this hereditary trait have extra copies of the tryptase gene and therefore permanently somewhat higher values, sometimes with symptoms and sometimes without. According to the Dutch Society for Clinical Chemistry (NVKC), a value above 20 µg/L may point to mastocytosis; this always requires further testing, usually at a center of expertise. Symptoms that may fit overactive mast cells or too many mast cells are episodes of flushing, itching, hives, abdominal pain, diarrhea, dizziness and severe reactions to a wasp or bee sting. Shortly after a serious allergic reaction, tryptase is raised for only a few hours; a later measurement no longer says anything about it.
Low
A value within the normal limits is the most common and gives no reason for further testing. A low value has no meaning on its own. Note: according to the Dutch medical specialists' guideline (FMS) on mastocytosis, a normal basal tryptase does not rule out systemic mastocytosis in adults, because some people with that diagnosis have a normal or even low value.
If your result is abnormal
With a raised resting value, the GP usually repeats the measurement first, because a one-off result can vary. If the value stays raised and there are symptoms, referral to an allergist, dermatologist or internist (specialist in internal medicine) follows, in the Netherlands often to a center of expertise (UMCG or Erasmus MC). There, further testing follows, such as DNA testing in blood and, if needed, a bone marrow biopsy.
Reliability
Tryptase is a supporting test, not a diagnosis. The Dutch medical specialists' guideline (FMS) on mastocytosis states that the right cut-off value is still under discussion and that a normal value does not rule out mastocytosis. A raised value is also not specific: it also fits hereditary alpha tryptasemia, in which the criteria for mastocytosis are not met. The diagnosis of mastocytosis is made with further testing, such as urine testing for histamine breakdown products, DNA testing for the KIT D816V mutation and usually a bone marrow biopsy.
Limitations
- After a severe allergic reaction, tryptase is raised for only a few hours; for that question, the blood must be taken shortly after the reaction and later compared with the resting value.
- A hereditary raised value (hereditary alpha tryptasemia) cannot be distinguished from other causes with this test; that requires DNA testing.
- According to the Dutch Society for Clinical Chemistry (NVKC), tryptase is usually normal in cutaneous mastocytosis, so a normal value does not rule out that form.
- Reference values and measurement methods differ between laboratories.
How common is this
Mastocytosis is rare. According to the UMCG, about 10 to 20 people in the Netherlands are diagnosed each year and a total of about 750 to 1000 adults and children are treated for it; Erasmus MC estimates the number of people with the most common form, indolent systemic mastocytosis, at about 2500. Children usually only have the skin form, which often disappears around puberty. A slightly raised tryptase without mastocytosis is clearly more common than mastocytosis itself.
Who is this useful for
Useful with symptoms that may fit overactive mast cells, such as recurring episodes of flushing, itching and hives, or after a serious allergic reaction without a clear cause, for example after an insect sting. Also useful with spots on the skin that fit mastocytosis. In people without such symptoms, a stand-alone tryptase measurement adds little: the chance of an abnormal result without disease is then greater than the chance of a real finding.
Downsides and risks
- A slightly raised value is often harmless or hereditary; still, such a result can trigger a lot of worry and invasive follow-up testing.
- A normal value can give false reassurance: it does not rule out mastocytosis and other mast cell diseases.
Sources
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results