Food allergy is a growing public health concern and its prevalence is increasing worldwide. The most common food allergens—milk, egg, soy, wheat, peanut, tree nuts, seeds, fish, and shellfish—account for more than 90% of food allergens.
Immunoglobulin E (IgE)-mediated reactions involve allergens binding to IgE antibodies; the consequential degranulation of mast cells releases several mediators, including antihistamines. Reactions typically present with acute symptoms and are reproducible upon repeated exposures. Organ systems affected by symptoms involve the skin (eg, urticaria/hives, pruritus, flushing), respiratory tract (eg, cough, wheeze, throat tightening or irritation, sneezing), the gastrointestinal tract (eg, nausea, vomiting, abdominal pain), cardiovascular system (eg, tachycardia, hypotension, dizziness, anaphylactic shock), neurologic system (eg, anxiety or “sense of impending doom”), or ocular (eg, conjunctivitis). Symptoms usually present within minutes up to the following 1 to 2 hours.
Unnecessary food avoidance can result in nutritional deficiencies and have severe social impact on patients. Therefore, it is extremely important to make an accurate diagnosis. Clinical history is the most important tool. Specific, allergy-focused elements of the history including the amount ingested, form of the food (raw or baked), time to reaction, specific symptoms, and prior exposures are all important to account for. Presence of other atopic conditions, such as severe eczema, asthma, and allergic rhinitis can also be clues.
Diagnostics in food allergy include serum specific IgE testing, skin prick testing, and oral food challenges. Double-blind, placebo-controlled oral food challenges are the gold standard for diagnosis. Another approach, component-resolved diagnostics, refers to specific IgE testing that is targeted toward different epitopes and can offer more diagnostic value in some situations.
It is important to know that testing or history alone are not diagnostic. Food allergy testing has a very high negative predictive value and sensitivity, but low specificity. A positive IgE or skin prick test result does not prove an allergy, and false positives can be seen between 30% and 60% of the time, depending on the study. False negatives are rare, and when food allergy is suspected but there is negative testing, then an oral food challenge can be diagnostic.
Broad food allergy IgE panels or skin testing to a full panel of food allergens are never recommended. These tests often lead to confusion and unnecessary dietary avoidance as well as both social and financial stress to the patient and family.
Periodic reassessment with skin or IgE testing can be performed to re-evaluate the allergy status and help determine if a patient has outgrown their allergy. Usually, an oral food challenge is completed in the office setting and patients are advised to continue to include the food as a regular, ad libitum part of their diet.