Clinical Information

N-methylhistamine (NMH) is the major metabolite of histamine, which is produced by mast cells. Increased histamine production is seen in conditions associated with increased mast-cell activity, such as allergic reactions, but also in mast-cell proliferation disorders, particularly mastocytosis.

Mastocytosis is a rare disease. Its most common form, urticarial pigmentosa (UP), affects the skin and is characterized by multiple persistent small reddish-brown lesions that result from infiltration of the skin by mast cells. Systemic mastocytosis is caused by the accumulation of mast cells in other tissues and can affect organs such as the liver, spleen, bone marrow, and small intestine. The mast-cell proliferation in systemic mastocytosis can be either benign or malignant. In children, benign systemic mastocytosis tends to resolve over time, while in most adults, the disease is progressive. Systemic mastocytosis may or may not be accompanied by UP.(1,2) Patients with UP or systemic mastocytosis can have symptoms ranging from itching, gastrointestinal distress, bone pain, and headaches; to flushing and anaphylactic shock.

Definitive diagnosis of systemic mastocytosis is made by bone marrow biopsy; however, patients with systemic mastocytosis also usually exhibit elevated levels of NMH.(1-5) Other biochemical markers include 11-beta prostaglandin F2 alpha, a metabolite of prostaglandin D2 (23BPT / 2,3-Dinor 11 Beta-Prostaglandin F2 Alpha, 24 Hour, Urine) and tryptase, alpha or beta (TRYPT / Tryptase, Serum). Histamine in blood or urine is also sometimes measured, but it generally has less diagnostic value than NMH measurement in urine, particularly if measurements in blood are not undertaken during a spell.