What test for what age. A few rules can help determine the choice of allergy test. For a patient younger than 2 years with noticeable atopic dermatitis (eczema), a skin test for food allergy is an ideal choice. In nearly 70% of these patients, an offending food will be identified.16,17 Consider an allergy test in children with a history of drug,3,18 latex,7,19 or insect sting allergy regardless of age.8,20
Because the IgE in blood or on mast cells is lower in children, neither RAST nor skin tests for airborne allergens may produce meaningful results until a child is 5 years old. However, in children 5 years and younger, it is prudent to test for indoor allergens, such as house dust mites, molds, animal dander, feathers, and cockroaches. In schoolaged children (6 years and older), it is helpful to test for outdoor allergens, such as grass, tree, and weed pollens.21,22
For children with urticaria or angioedema, allergy testing is left to the discretion of the clinician, because often the search for an allergic cause is not fruitful.19,20 A summary of skin test reagents for different ages is provided in Table 2.
Parents are always concerned about the volume of blood needed for in vitro allergy testing; thus, the skin test remains an attractive choice in younger children.
ORAL FOOD CHALLENGE
With the number of children with food allergy increasing worldwide,5 it has become more of a challenge to distinguish food allergy from other food-induced reactions. Reactions to food include both true food allergy (IgE-mediated, non–IgE-mediated, or a combined reaction in which both IgE- and non–IgE-mediated mechanisms are involved) and food intolerance (metabolic, pharmacological, or idiosyncratic). The known non–IgEmediated food allergies include food protein–induced enteropathy; food protein–induced enterocolitis; food protein–induced proctitis; celiac disease; and allergic eosinophilic esophagitis, gastritis, and gastroenteritis.
A positive IgE test result for food sensitivity is by itself insufficient to arrive at the diagnosis of food allergy. When the offending food has not been identified despite a thorough history and attempt to document sensitization or when multiple foods are implicated, the oral food challenge is used to determine which of the suspected foods is causing symptoms.9 When something other than an IgEmediated sensitivity is suspected, an oral food challenge may be the only accurate means of verifying the diagnosis. Before an oral food challenge, it is appropriate to enforce an elimination diet to see whether the patient's symptoms decrease.
Oral food challenges are categorized into open, single-blind placebocontrolled, and double-blind placebocontrolled. The last of these methods is the one currently recommended. The foods suspected of causing allergy are introduced slowly and steadily, with 30 minutes between each feeding. Pediatric clinicians who attempt an oral food challenge must have access to emergency rescue equipment in case of a reaction. Generally, this test is best performed in an allergy specialist's office or in an inpatient service.