Anaphylaxis is a serious allergic reaction that develops rapidly and can cause death; diagnosis is based on specific criteria (Table 3).27 Data for rates of anaphylaxis due to food allergy are limited. The incidence of fatal reaction due to food allergy is estimated to be 1 in every 800,000 children annually.3
Clinical suspicion. Food allergy should be suspected in infants and children who present with anaphylaxis or other symptoms (Table 225) that occur within minutes to hours of ingesting food.4 Parental and self-reports alone are insufficient to diagnose food allergy. NIAID guidelines recommend that patient reports of food allergy be confirmed, because multiple studies demonstrate that 50% to 90% of presumed food allergies are not true allergy.4 Health care providers must obtain a detailed medical history and pertinent family history, plus perform a physical exam and allergy sensitivity testing. Methods to help diagnose food allergies include skin-prick tests, allergen-specific serum IgE tests, and oral food challenges.4
General principles and utility of testing
Before ordering tests, it’s important to distinguish between food sensitization and food allergy and to inform the families of children with suspected food allergy about the limitations of skin-prick tests and serum IgE tests. A child with IgE antibodies specific to a food or with a positive skin-prick test, but without symptoms upon ingestion of the food, is merely sensitized; food allergy indicates the appearance of symptoms following exposure to a specific food, in addition to the detection of specific IgE antibodies or a positive skin-prick test to that same food.28
Skin-prick testing. Skin-prick tests can be performed at any age. The procedure involves pricking or scratching the surface of the skin, usually the volar aspect of the forearm or the back, with a commercial extract. Testing should be performed by a physician or other provider who is properly trained in the technique and in interpreting results. The extract contains specific allergenic proteins that activate mast cells, resulting in a characteristic wheal-and-flare response that is typically measured 15 to 20 minutes after application. Some medications, such as H1- and H2-receptor blockers and tricyclic antidepressants, can interfere with results and need to be held for 3 to 5 days before testing.
A positive skin-prick test result is defined as a wheal ≥ 3 mm larger in diameter than the negative control. The larger the size of the wheal, the higher the likelihood of a reaction to the tested food.29 Patients who exhibit dermatographism might experience a wheal-and-flare response from the action of the skin-prick test, rather than from food-specific IgE antibodies. A negative skin-prick test has > 90% negative predictive value, so the test can rule out suspected food allergy.30 However, the skin-prick test alone cannot be used to diagnose food allergy because it has a high false-positive rate.
Continue to: Allergen-specific serum IgE testing