The Test Sample
What is being tested?
Histamine is a substance that is released from specialized cells called mast cells when they are activated, often as part of an allergic immune response. This test measures the amount of histamine in the blood or urine.
Mast cells are large tissue cells found throughout the body. They are present mainly in the skin, the lining of the intestine and air passages, and the bone marrow. Mast cells are part of the body's normal response to injury as well as allergic (hypersensitivity) responses. They contain granules that store a number of chemicals, including histamine and tryptase, which are released when mast cells become activated. Histamine is responsible for many of the symptoms in persons with allergies.
Concentrations of histamine in the blood and urine are normally very low. Significant increases can be seen in people with a severe allergic reaction and in those with a disorder in which the number of mast cells increase (proliferate) and/or activate without apparent allergies.
The activation of many mast cells is associated with a severe form of acute allergic reaction termed anaphylaxis, which can cause hives (blisters on the skin), reddening of the skin (flushing), low blood pressure, severe narrowing of the air passages, and even death. With anaphylaxis, histamine concentrations in the blood increase rapidly, rising within 10 minutes of the start of symptoms and returning to normal within about 30 to 60 minutes. This increased production is also reflected a short time later in the urine as histamine and its primary metabolite, N-methylhistamine, are excreted.
Histamine and tryptase levels may be persistently increased in people with mastocytosis. This rare condition is associated with abnormal proliferation of mast cells and their infiltration and accumulation in the skin (cutaneous mastocytosis) and/or in organs throughout the body (systemic mastocytosis).
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm. For a 24-hour urine collection, all of the urine should be saved for a 24-hour period. It is best to keep the sample in a cool, dark place such as a refrigerator.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
If anaphylaxis is suspected, it is important to collect the sample very soon after the beginning of symptoms. If testing is done for other conditions, you may be instructed to refrain from taking antihistamine and other medications. This should be discussed with your doctor.