ASCERTAINING SHELLFISH ALLERGY

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An adult patient who has eaten shellfish regularly throughout his life suddenly developed swelling after eating shrimp in a restaurant. What is the best way to determine if this is a true allergy to shrimp? Will the daily use of second-generation antihistamines help, and is this an acceptable approach? The only relevant history was that the patient had received allergy shots for environmental allergies when he was a teenager.
— Miguela Gutierrez, MD, Williamsburg, Va.

Food-allergy symptoms result from the release of immunoglobulin (Ig) E by mast cells in response to an allergen, usually within an hour of exposure, so a history of rapid symptom development after ingestion of a likely cause (peanuts, tree nuts, shellfish, and fish in adults) is highly suggestive. The most practical methods of diagnosing a food allergy are to perform radioallergosorbent testing (RAST) or skin-prick testing for IgE. Both tests have high negative predictive value but low positive predictive value, although a positive test with a typical allergy history is sufficient to make the diagnosis. You can tailor the testing to the suspected foods. If the history is in doubt, the double-blind, placebo-controlled food challenge remains the gold standard for food-allergy diagnosis. Small amounts of the suspected food are given to the patient in a controlled, blinded, and supervised setting. The mainstay of treatment for food allergies is avoidance of the food and its proteins and the implementation of an emergency plan for accidental ingestions; this should include injectable epinephrine and oral antihistamines. The use of daily antihistamines is not currently recommended except for prevention of chronic urticaria, which may rarely be associated with food additives.
—Seonaid Hay, MD, assistant professor of medicine, Yale University School of Medicine, New Haven, Conn. (104-9)

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