The importance of checking for delayed reactions in pediatric patch testing

Patch testing is often performed with the final reading being completed at 72 hours. Studies in adults have shown that performing an additional reading several days later increases the yield of relevant positive reactions. We report a study of this matter in children. We conducted a prospective patch test study of 38 affected children aged 6 to 17 (mean 11.6 years), with presumed allergic contact dermatitis between April and July 2009. The patients were required to return at 48 hours for patch test removal and assessment of early reactions, at 72 to 96 hours (3-4 days) for evaluation of delayed reactions, and again at 168 to 216 hours (7-9 days) for a final late delayed reading. We tallied the number of patients with "equivocal" (+/?), +, ++, and +++ reactions and noted these as potential positive reactions by reading date. Twenty-five of the 38 children (66%) had a positive reaction at 48 hours; 32 children (84%) had a positive reaction at 72 hours (day 3); 19 children (50%) had a positive reaction at 168 to 216 hours (day 7-9). Of those 19, 16 (42%) had persistent reactions, while 5 children (13%) had new late delayed reactions. Among the new late delayed reactions, there were six allergens identified, four of which were considered of probable clinical relevance. When patch testing with Thin Layer Rapid-Use Epicutaneous test panels, a delayed reading beyond 72 hours may be needed to identify the offending agent(s) responsible for allergic contact dermatitis in some children. Additional studies are needed to assess optimal readings schedules.