Useful for evaluating for diseases of immediate hypersensitivity or mast cell proliferation (mastocytosis). Histamine is one of the major mediators of allergic reactions. Some carcinoid tumors, particularly of gastric origin, produce and release excessive histamine. Symptoms of flushing, itching, urticaria, vomiting, syncope or shock assumed to be due to histamine release, which coincides with increased urine of plasma levels. Systemic mastocytosis or basophilia can produce persistent elevations in plasma or urine histamine concentrations.
Significantly elevated histamine and/or tryptase levels in a person with symptoms of anaphylaxis (severe, potentially life-threatening allergic reaction) are strong evidence for that diagnosis.
Normal histamine results may indicate that a person's symptoms are due to another cause, or that the sample was not collected at the right time. With anaphylaxis, blood histamine levels rise rapidly and can fall back to normal within about 30-60 minutes. If a sample is drawn too late, results may be normal. If a tryptase test is also performed, its value can be compared to the histamine levels. Tryptase levels rise and fall more slowly than histamine levels, peaking within 1 to 2 hours of symptom development.
If the timing of sample collection was appropriate and neither the blood histamine or tryptase concentration is elevated, it is less likely that a person had anaphylaxis. However, a person can have anaphylaxis or mastocytosis without elevated histamine levels, so the diagnosis cannot be ruled out just because the test is negative.
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