Tryptase is an enzyme (protein) that is released along with histamine and other chemical substances by mast cells, the white blood cells responsible for allergic reactions. Histamine and other chemical substances are responsible for the sometimes very severe symptoms of an allergic reaction (anaphylactic shock). Results are expressed in micrograms of tryptase per litre of blood (µg/L).
Normal levels of tryptase indicate that the individual’s symptoms are probably not due to mast cell activation. Levels of tryptase typically reach their maximum level one to two hours after the onset of symptoms. The time of collection is therefore important to prevent false negative results. High levels found intermittently or acutely in an individual with anaphylaxis symptoms indicate that an allergic reaction is most likely the cause. Chronically high levels of tryptase suggest either a mast cell activation disorder or mastocytosis, a rare disease associated with an abnormal increase in the number of mast cells. They can accumulate in the skin (cutaneous mastocytosis) leading to urticaria pigmentosa, or in all organs (systemic or malignant mastocytosis, mast cell leukemia).