Histamine is a mediator of the allergic response. Histamine release causes itching, flushing, hives, vomiting, syncope, and even shock. In addition, some patients with gastric carcinoids may exhibit high concentrations of histamine. This test aids in evaluation of patients with allergic signs and symptoms, i.e. anaphylaxis. May also assist when diagnosing and monitoring mast-cell activation disorders or when evaluating histamine production over a longer time frame.