Atopy patch test

Abstract

Approximately 30 years ago, atopic dermatitis (AD) patients were patch tested with aeroallergens, in order to experimentally reproduce the normal way of exposure of atopic skin to environmental allergens. This procedure was later called "atopy patch test" (APT). So far, many authors have performed APTs with house dust mites, pollen and animal dander in AD patients, obtaining different positivity rates (15-100%) according to patch test materials and modalities. Positive skin prick tests (SPTs) and/or high level of specific immunoglobulin E (IgE) in serum are not a prerequisite for a positive APT response. APTs have been demonstrated to improve the accuracy of skin testing in the diagnosis of allergy in AD patients; whereas immediate-type reactions proved to be associated with SPT positivity, APT reactivity is more frequently observed in patients with delayed responses. Combined prick and patch testing proved to enhance identification of food allergy in AD patients and to help in prescribing elimination diets. APTs are believed to be a useful diagnostic procedure also in patients with digestive symptoms (diarrhea, gastro esophageal reflux, constipation, colics, and eosinophilic esophagitis) or respiratory symptoms (rhinitis and asthma). Positive APT responses were observed also in healthy subjects. This finding may indicate an atopic diathesis and identify patients predisposed to develop dermatitis on allergen exposure. About 20 μg of patch test material are applied to the back with Large Finn Chambers on Scampor tape for 48-72 hours. Readings are performed according to ETFAD (European Task Force on Atopic Dermatitis) guidelines, taking into account erythema, infiltration, number of papules and presence of vesicles. As regards food, APTs can be carried out with self-made fresh food material, prepared each day before patch testing or, recently with commercial food extracts. The methodology of food APTs is not standardized so far: great variations in the allergenic content of food materials from different sources may be expected, and this may influence the results and give rise to a poor reproducibility. Moreover, relevance of positive and negative APT responses should be assessed by food challenge results, as for SPT responses. Pretreatment with tacrolimus ointment does not inhibit the APT reaction in patients with DA, whereas antihistamines, systemic and topical steroids should be discontinued before patch testing. APTs should not be performed in pregnant women, in tanned subjects and in AD patients with diffuse and and/or acute eczema. Adverse side-effects were reported, described as local eczema flares, contact urticaria, irritation caused by adhesives, and itching in test sites. Severe adverse effects from APTs are very rare.