The atopy patch test (APT) - a useful tool for the diagnosis of food allergy in children with atopic dermatitis

Background: While immediate-type clinical reactions to food can quite easil y be identified by history or measurement of specific IgE in combination wi th positive oral food challenges, the evaluation of food allergy in the abs ence of immediate clinical reactions still presents diagnostic difficulties - particularly in children with atopic dermatitis. The objective of this s tudy was to evaluate the diagnostic value of the atopy patch test (APT) wit h regard to late-phase reactions observed in double-blind, placebo-controll ed food challenges with cow's milk, hen's egg, wheat, and soybean. Methods: We investigated 75 children (median age 2.1 years) with suspected food allergy by double-blind, placebo-controlled food challenges, specific IgE in serum, skin prick test, and APT. Of the subjects, 69/75 suffered fro m atopic dermatitis. Results: Of 209 oral challenges, 133 were performed with allergen and 76 wi th placebo. We assessed 77/133 allergen and 2/76 placebo challenges as posi tive. In 66 of 77 (86%) positive oral challenges, specific IgE in serum to the corresponding allergen was positive; in 64/77 (83%) the skin prick test , and in 42/77 (55%) the APT was positive. While immediate-type reactions w ere associated with positive skin prick test and proof of specific IgE in s erum, late-phase clinical reactions were associated with a positive APT (se nsitivity 76%, specificity 95%). Conclusions: The APT seems to be a valuable additional tool in the diagnost ic work-up of food allergy in children with atopic dermatitis - especially with regard to late-phase clinical reactions. The APT may help to prevent u nnecessary restrictive diets which may be the consequence of misjudging lat e reactions by clinical assessment alone.