Explanation to Question 2
Skin prick testing involves introduction of allergen extracts into the skin. A positive reaction is defined as a wheal at least 3mm greater than the negative control. The negative predictive value is > 95% while the positive predictive value is <50%; therefore, there are many false positive results. Antihistamines should be discontinued prior to testing. Testing cannot be performed on skin with extensive eczema/rash or in patients with dermatographia. In these patients, serum specific IgE testing may be of use. Serum specific IgE testing is the detection of the serum specific IgE to specific allergens. All positive and negative tests need to be correlated with the patient’s clinical history. A positive test alone does not make the diagnosis of clinical food allergy. Rather, it provides evidence of sensitization, i.e., an immunological response.
A double-blind, placebo controlled food challenge is the gold standard for diagnosis of food allergy, though open challenges are generally used in most clinical settings. The patient is given gradually increasing amounts of the suspected food allergen over a time period of hours to a day. The process requires close physician supervision. Patients may benefit from an oral food challenge if they have borderline test results or if a false positive or false negative is suspected based on clinical history. Oral food challenges may prevent unjustified food elimination from the diet.
Atopy patch tests are currently used to diagnose delayed hypersensitivity T-cell mediated reactions such as contact dermatitis. Atopy patch testing for IgE-mediated food allergy is not recommended.
The correct answer is B, serum specific IgE testing or skin prick testing are appropriate initial tests for food allergy.
Sources:
1. Sampson H. Update on food allergy. J Allergy Clin Immunol 2004; 113:805-19.
2. American College of Allergy, Asthma, & Immunology. Food allergy: a practice parameter. Ann Allerg Asthma Immunol 2006; 96:S1-68.
