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A 24-year-old woman comes to your office for evaluation 2 days after an emergency department visit. She says that she went to the emergency department because of an œallergic reaction that consisted of itching in the throat after eating ice cream, followed by nausea and vomiting, a sensation of flushing, and then hives (urticaria). The same kind of thing happened to her last year after eating chocolate candy, and once before that in a Thai restaurant. Her friend suggested that this might represent a peanut allergy, but the patient does not recall eating peanuts when she had these reactions. When she is in your office all manifestations of the allergic reaction have resolved. The patient seems healthy and has no significant past medical history. Her only medication is oral contraceptives. Physical examination in your office is normal. Which of the following is an appropriate next step in evaluating and managing the patient?
A. Begin desensitization (allergy shots) with peanut extract
B. Have the patient keep a food diary to identify food consumed before the next allergic episode
C. Initiate a food challenge with peanuts
D. Order a skin-prick test or radioallergosorbent test for peanut allergy
E. Start the patient on preventive therapy with H-2 blockers and diphenhydramine
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The correct answer is D. This patient™s history is compatible with peanut-induced food allergy. Peanut allergy is an IgE-mediated reaction with typical symptoms of pharyngeal itching, flushing, and urticaria. Many patients also have nausea and vomiting or asthma-like wheezing, and some develop potentially fatal angioedema. Although a history of allergic reaction immediately following peanut ingestion is helpful in making the diagnosis, such a history is often absent. Instead, many patients experience symptoms following consumption of other foods that contain peanut oil or residues, often simply because those foods have been in contact with peanuts or prepared in the same cookware as peanut-containing food or peanut oil. Some of the most common of these foods are nuts that are sold as nuts other than peanuts, but that are really peanuts, and ethnic foods that often contain peanuts (African, Chinese, Indonesian, Mexican, Thai, and Vietnamese dishes). Foods sold in bakeries and ice cream shops, particularly chocolate candies, are also often in contact with peanuts or peanut residues. If the history does not clearly indicate peanut allergy, confirmation of the diagnosis can be made either with a skin-prick test or a blood radioallergosorbent test, both of which are positive if patients have IgE antibodies against peanuts. A positive radioallergosorbent test indicates a 95% likelihood that the patient has peanut allergy.
Desensitization with allergy shots (choice A) is incorrect for two reasons. The main reason is that in contrast to desensitization with other allergens, the risk-benefit ratio of desensitization with injections of peanut extract is too high; there is a substantial risk for inducing a life-threatening allergic reaction. The second reason is that even if desensitization were safe, it would not be an appropriate therapy until the diagnosis of peanut allergy is confirmed.
Keeping a food diary (choice B) might be helpful if the patient did not know what foods induced the allergic reaction. In this case, however, the patient is able to provide reasonably clear information about the foods that preceded her allergic reaction.
A food challenge with peanuts (choice C) is appropriate if the diagnosis is not clear from the history and from skin-prick or radioallergosorbent tests. Food challenge carries risk for inducing anaphylaxis, so it should be performed under medical supervision. Furthermore, because of the risk for anaphylaxis, a food challenge should not be performed unless the diagnosis is uncertain and not confirmed by other means.
Starting the patient on H-2 blockers and diphenhydramine (choice E) is also incorrect. Patients with known peanut allergy should be given a written emergency plan that can be implemented if they unexpectedly ingest peanuts and experience allergy symptoms. The plan includes self-administration of appropriate doses of diphenhydramine and self-injectable epinephrine to use in the event of severe allergy. Choice E is incorrect partly because it does not include epinephrine. It is also incorrect because the diagnosis of peanut allergy has not yet been confirmed, and it would be inappropriate to send the patient home with allergy treatments without first confirming the nature of the patient™s allergy.
