Mast cells are present mainly in the skin, the lining of the intestine and air passages and the bone marrow. They contain granules that store 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 (plasma) 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.
Histamine and tryptase levels may be persistently increased in people with MCAS, MCAD and mastocytosis. This 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) and or an increase reactivity to histamine.
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.
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 healthcare practitioner.