Food atopy patch test and repeated food challenge

Food hypersensitivity is a common medical problem in atopic dermatitis (AD) patients, mainly in paediatric age. Patch tests based on prolonged exposure of the skin to food, aiming at the detection of delayed reactions, represent a testing modality reproducing skin responses against allergens normally occurring in AD. Atopy patch tests (APTs) have been demonstrated to improve the accuracy of skin testing in the diagnosis of food allergy in AD patients; whereas immediate-type reactions proved to be associated with skin prick test positivity, APT reactivity is more frequently observed in patients with delayed responses. Relevance of positive and negative APT responses should be assessed by food challenge results. Double-blind placebo-controlled food challenge (DBPCFC) is considered the gold standard for diagnosing food allergy, however, besides the fact that there is no universally accepted standard for performing the DBPCFC, these testing and observation modalities may not identify the whole spectrum of skin reactions possibly appearing in AD patients after food ingestion, which encompasses both prompt and eczematous responses sometimes appearing more than 48 hours after the challenge and requiring repeated administration of the foodstuff. Since many authors demonstrated that an open challenge associated to careful follow-up can prove adequate in identifying food allergic patients, also enabling the diagnosis of delayed reactions, the repeated open food challenge (ROFC) may be considered a useful and practical alternative to DBPCFCs. In our experience this challenge modality seems to be well accepted and enables a good compliance. The subjects undergoing challenge are given a dose of the suspected food reflecting the normal intake daily at home for a week. The first administration is performed at the hospital in selected cases, depending on the nature and severity of the reaction to food allergens. When a clinical reaction is noted, the test is stopped and the patient is examined at the hospital. All subjects are examined on day 7 of the challenge. APTs with egg, cow’s milk and peanut were performed in 222 AD subjects undergoing repeated open challenges. 11%, 9% and 22% of them reacted to APT with egg, milk and peanut, respectively. Whereas sensitivity figures were low (31% for egg, 15% for cow’s milk, and 37% for peanut), specificity was high (79% for egg, 92% for cow’s milk, and 77% for peanut), indicating that a positive patch test enables the identification of food-allergic subjects.