Patch testing in allergic contact dermatitis: is it useful to perform the cosmetic series in addition to the European standard series?

Background Cosmetics are the causative agents in 8-15% of patients suspected of having allergic contact dermatitis. Patch testing with standard series identifies 70-80% of the responsible allergens in all contact dermatitis; however, many important cosmetic-related allergens may be missed by using standard series alone. Objective The aim of this study was to determine the value of using cosmetic series in addition to the European standard series in patients with suspected allergic contact dermatitis. Methods In this prospective study, 93 consecutive patients suspected of having allergic contact dermatitis were patch tested with the European standard series, and simultaneously with cosmetic series. Positive allergic reactions were further interpreted as clinically relevant or irrelevant. The clinically relevant reactions were subsequently stratified into three subgroups: (i) reactions only to allergen/allergens in the European standard series; (ii) reactions only to allergen/allergens in cosmetic series; and (iii) reactions both to allergen/allergens in the European standard and cosmetic series. Results A total of 74 positive reactions were observed in 93 patients. However, only 46 (62.2%) of the total positive reactions were found to be clinically relevant. Of all the clinically relevant positive reactions, 27 (58.7%) were caused by the allergens in the European standard series; 19 (41.3%) were caused by the allergens in cosmetic series. Of the 93 patients tested, 44 (47.3%) had at least one positive allergic reaction, 30 (68.2%) of whom had clinically relevance. Of the 30 patients with clinically relevant positive tests, 16 (53.3%) reacted only to allergens in the European standard series; nine (30%) reacted only to cosmetic series allergens; and five (16.7%) reacted both to the European standard and cosmetic series allergens. Among the 45 cosmetic series allergens tested, 15 (33.3%) gave positive reactions of which 14 (93.3%) of those were found to be clinically relevant. The clinically relevant cosmetic series allergens which were found to be over the critical incidence of 1% included methyldibromo glutaronitrile, Euxyl K400, and isopropyl myristate. Conclusion Patch testing with cosmetic series in addition to the European standard series increased the capability to detect the relevant allergen/allergens, particularly in patients with a suspicion of cosmetic allergy. However, it is not practical and cost-effective to test those patients routinely with all 45 allergens in the cosmetic series. As the European baseline series which includes methyldibromo glutaronitrile is now widely used as the guideline minimum set of allergens for routine diagnostic patch test investigations, we additionally recommend Euxyl K400 and isopropyl myristate as the candidates for patch testing.