24-Hour versus 48-Hour Occlusion in Patch Testing

Background: Routine patch testing is usually performed with 2-day occlusion. Yet, shortening the application period, without diminishing diagnostic accuracy, would benefit patients. Previous studies document that differences in application time may produce variable results. Objective: Our objective was to determine whether 1-day application elicits similar results to 2-day application in diagnostic patch testing. Methods: Simultaneous duplicate patch test series were applied to the opposite sides of the upper back on 250 consecutive patients, using the TRUE test system. One set of allergens was removed after 24 and the other after 48 h. Results: A total of 221 positive reactions were observed either after 24 or 48 h of occlusion in 113 (46%) patients. Of these, 190 were concordant, i.e. positive, after 24 and 48 h of occlusion, and 31 were discordant. Twenty-nine of the discordant reactions were observed only after 48 h of occlusion and 2 only after 24 h of occlusion. Of the 27 (19%) patients with discordant patch test reactions, the allergen was deemed to be of definite present or past relevance in 16 (6% of the total and 14% of all patients with positive reactions). Conclusion: These data confirm that the overall concordance of results after 24 and 48 h of application is high. However, clinically relevant allergens would have been missed in 16 patients if only the 24-hour occlusion test was performed. In the light of these results, we consider that the standard 48-hour application remains appropriate for diagnostic patch testing.