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Allergy Testing in Children: Which Test When?

Allergy Testing in Children: Which Test When?

ABSTRACT: For the child with persistent or worsening allergy symptoms of more than 2 months' duration, a screening radioallergosorbent test may be initiated after a careful history and physical examination. In the workup, be alert for allergic "shiners" and other signs of allergic disorders (eg, a bulging chest wall is suggestive of chronic asthma). Skin testing, performed on the skin of the back, is the most commonly used allergy test. In patients younger than 2 years with atopic dermatitis (eczema), a test for food allergy frequently identifies an offending food. It is prudent to test for indoor allergens in children younger than 5 years and to delay testing for outdoor allergens until 6 years or older. Consider an allergy test in children with a history of drug, latex, or insect sting allergy regardless of age. Further testing (eg, with an oral food challenge) can be done at the clinician's discretion. Because such tests are timeconsuming and require assistance, referring the patient to an allergy specialist is recommended.

Allergy testing can aid the diagnosis of allergic disorders; however, it is not diagnostic. With skin testing, in particular, a positive result does not necessarily indicate clinical allergy, and a negative result does not always exclude clinical relevance. Allergy test results serve only as confirmation of the accuracy of the patient's history and physical examination findings and should be used with discretion. In the best-case scenario, allergy test results can offer a means for advising the patient and parents to avoid offending allergens that can be potentially life-threatening. They can also provide a basis for immunotherapy in cases of drug allergy (rapid desensitization) or respiratory allergy (longterm allergy desensitization).

In this article, I provide a step-by-step guide for the evaluation of children with allergy symptoms. The focus is on the types of allergy tests available and their advantages and limitations, which tests to perform for each age-group, and when referral to an allergy specialist is appropriate.

ALLERGY DEVELOPMENT

The development of allergy involves the formation of the IgE antibody: the higher the IgE antibody level, the greater the manifestation of allergy symptoms. IgE formation depends on complex genetic and environmental influences,1 which are only now being elucidated. The presence of IgE alone does not necessarily lead to allergic reactions.

Allergy develops in 2 steps: sensitization, in which IgE attaches to mast cells that reside under the surface of tissues of the respiratory tract, GI tract, and skin; and re-exposure of the sensitized person to the allergen (or allergic factors). When enough allergens bind to the IgE that is already present on the mast cells, the mast cells become activated and release mediator granules into the surrounding tissues. An allergy is the response of the tissue to these mediators. The most common mediators—histamine, plateletactivating factor, leukotrienes, and prostaglandin D—increase vascular permeability, dilate vessels, contract smooth muscle, cause bronchospasm, and summon inflammatory cells. Most IgE-mediated reactions occur within minutes to a few hours after exposure (although the manifestations of non–IgE-mediated reactions— mostly GI—may be delayed for several hours or even days).

Allergic disorders are classified as follows, depending on the primary target organ: allergic asthma, allergic rhinitis, allergic conjunctivitis, urticaria, angioedema, atopic dermatitis (eczema), food allergy, insect sting allergy, drug allergy, and latex allergy. The most severe allergic disorder, anaphylaxis, can simultaneously involve the respiratory, cardiovascular, and GI systems and may be caused by allergies to food, drugs, insect stings, or latex. Fatalities have been reported with all of these disorders.

TYPES OF ALLERGY TESTING

Because of the role of IgE in the evolution of allergy symptoms, the most convenient assay used in clinical practice is the measurement of IgE.2-10 The next most commonly used tool is the oral food challenge, 11,12 in which suspected food allergens are introduced in a graded fashion to determine whether allergy symptoms can be reproduced. The third most common test is the measurement of the serum tryptase level to determine whether the patient has had an anaphylactic reaction.13,14

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