Although prick skin testing and the measurement of serum food-specific IgE antibodies are indispensable in the evaluation of patients with IgE-mediated food allergy, these tests are not useful in identifying the responsible food in patients with nonIgE-mediated reactions where cell mediated immune mechanisms are involved.

In an effort to identify a test that might be predictive in these situations, the use of the atopy patch test (APT) has been explored in recent years in the evaluation of patients with atopic dermatitis, eosinophilic esophagitis, food protein-induced enterocolitis syndrome (FPIES), and others with gastrointestinal symptoms suspected of being food-related, but not IgE-mediated.

The foods most commonly evaluated with the APT are milk, egg, soy, and wheat. The APT is performed by applying the intact food allergen to noninflamed skin on the back under occlusion in a small aluminium cup. After 48 hours the patch test is removed and the resulting reaction is assessed and recorded, initially at 20 minutes and again 24 hours after patch test removal.

The reactions are graded based on the degree of erythema and the presence of papules or vesicles. Thus, the APT has been referred to as an epicutaneous patch test using allergens capable of causing IgE-mediated reactions, where the test sites are evaluated for an eczematous response after 24 to 72 hours.

Although side effects are uncommon, irritant reactions and contact urticaria have been reported. The utility of the APT in the evaluation of food allergic patients remains a topic of debate.

At this point, the interest lies in the potential utility of this test as an aid in identifying the responsible foods in patients with nonIgE-mediated gastrointestinal disorders, such as FPIES, or mixed reactions involving both IgE and lymphocyte mediated mechanisms, as seen in eosinophilic esophagitis and atopic dermatitis.

Aspects that hinder wider use of the APT include the lack of standardization of the procedure, including the lack of standardized reagents or how best to prepare them, in addition to the time and expertise required for the accurate performance of the test.