The European baseline series (EBS) of contact allergens is subject to change. An allergen is considered for inclusion in the EBS when routine patch testing of patients with suspected contact dermatitis results in at least 0.5% prevalence rate. We aimed to determine the frequency of sensitizations to 40 allergens and the usefulness of late readings.

Patch testing with our baseline series of 40 allergens was done in 748 consecutive (unselected) adult patients (median age 45 years [IQR 32−59], 61.5% aged ≥ 40 years, 73.5% female) with suspected contact dermatitis who were evaluated at our tertiary referral center from January 23, 2019, to October 30, 2021. Our series of allergens contained 28 of 30 allergens that were eventually listed in the 2019 EBS panel (except 2-Hydroxyethyl methacrylate and Caine mix). Ten allergens not belonging to 2015 and 2019 EBS panel are highlighted in Table 1. The substances were provided by allergEAZE and Chemotechnique and applied with square allergEAZE chambers to the upper back for 48 hours. Patients removed the patches after that time. Readings were done on day 3 (D3) and D6 or D7 (D6/7). Clinical relevance of positive (pos.) reactions was not assessed. Data were routinely collected in an electronic databank and retrospective analysis of results wase done using IBM SPSS software version 25.

Frequencies of pos. reactions (n = 1005) are given in Table 1. At least one pos. reaction was found in 55.9% of patients. Proportion of sensitizations ≥ 0.5% was found for 35 allergens. Frequencies of pos. reactions demonstrated on D6/7 only (n = 167 [16.6% of pos. reactions]) are also shown in Table 1. Eight allergens not listed in 2015 nor 2019 EBS had ≥ 0.5% prevalence rate (ie, cocamidopropyl betaine, thimerosal, disperse blue mix (106/124), 2-bromo-2-nitropropane-1,3-diol, propylene glycol, compositae mix, diazolidinyl urea, and dexamethasone). Reactions on locations tested with these 8 allergens were most commonly weak (+) pos. (Table 1).

We propose further studies to evaluate whether the aforementioned 8 allergens should be included in the EBS. The possibility that some of the weak (+) pos. reactions could have been irritant should be taken into account. Our study also shows the importance of late readings since relatively high frequencies of pos. reactions would have been missed if such readings had not taken place.