SummaryThe present guideline summarizes all aspects of patch testing for the diagnosis of contact allergy in patients suspected of suffering, or having been suffering, from allergic contact dermatitis or other delayed-type hypersensitivity skin and mucosal conditions. Sections with brief descriptions and discussions of different pertinent topics are followed by a highlighted short practical recommendation. Topics comprise, after an introduction with important definitions, materials, technique, modifications of epicutaneous testing, individual factors influencing the patch test outcome or necessitating special considerations, children, patients with occupational contact dermatitis and drug eruptions as special groups, patch testing of materials brought in by the patient, adverse effects of patch testing, and the final evaluation and patient counselling based on this judgement. Finally, short reference is made to aspects of (continuing) medical education and to electronic collection of data for epidemiological surveillance.

Funding informationNo support was given by any medical company for development of this document. All meetings were virtual meetings, therefore there were no costs. All participants in the working group filled in a structured form to declare financial or non-financial interests. Disclosures are given in the Supporting information. The Guidelines were approved by the executive committee of the ESCD in

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Background: Fragrance mix II (FM II) is included in the baseline patch test series recommended by the International Contact Dermatitis Research Group (ICDRG). Hydroxyisohexyl 3-cyclohexene carboxaldehyde (HICC) is the most important sensitizer of the 6 fragrance materials included in FM II. Besides being a part of FM II, HICC is also tested separately in the ICDRG baseline series.Objectives: The aim of the study was to investigate the prevalence of contact allergy to FM II and HICC in 2012-2016 with a focus on simultaneous reactions and the percentage of missed contact allergy to HICC provided that only FM II had been tested.Patients and Methods: A total of 25,019 consecutive dermatitis patients in 13 dermatology clinics representing 12 countries in 5 continents were patch tested with FM II and HICC in the baseline series.Results: Contact allergy to FM II and HICC was found in 3.9% and 1.6%, respectively. For FM II, the frequency varied from 1.5% to 7.6% in different centers. The corresponding range for HICC was 0.2% to 3.6%. Simultaneous contact allergy to FM II and HICC was noted in 1.4% with the range 0.2% to 2.6%. Seventy-seven patients (0.31%) with contact allergy to HICC did not test positively to FM II. The range for missed HICC allergy by testing only FM II in the different centers would be 0.04% to 0.74%. The ratio between the contact allergy rates for FM II and HICC was similar for all centers, except for Montreal having significantly more contact allergy to FM II than to HICC.Conclusions: The frequency of missed contact allergy to HICC when testing only with FM II was less than 0.5%, therefore questioning the need to test HICC separately in the ICDRG baseline series.