Long Lasting Allergic Patch Test Reactions: A Literature Review.

Long-lasting allergic patch test reactions (LLAPTR) are those in which the clinical features of palpable erythema are still present at the site of a positive allergic patch test reaction 14 or more days after application of the allergen. LLAPTR have been described for a wide range of contact allergens, many of these included in the baseline patch test series. LLAPTR are far from uncommon; they occur in consecutive patients with positive patch tests to baseline allergens with frequency up to 17.9% of the total reactions. Patch test reactions persisting for a very long time (up to several months after the test) have been described, the most frequent ones being those induced by gold salts. The pathomechanisms of LLAPTR have not been clarified, but may hypothetically involve a constant antigen stimulation and/or a defect in cell-mediated immunity down-regulation. Host-related factors significantly associated with LLAPTR are, according to some studies, a strong initial patch test response, older age, and atopy. No significant sex differences have been observed in the frequency of LLAPTR.