Suitable test concentration of cobalt and concomitant reactivity to nickel and chromium. A multicentre study from the Swedish Contact Dermatitis Research Group

Background: In Sweden, cobalt chloride 0.5% has been included in the baseline series since the mid‐1980s. A recent study from Stockholm showed that cobalt chloride 1% petrolatum (pet.) was more suitable than 0.5%. Cobalt chloride at 1.0% has been patch tested for decades in many European countries and around the world. Objectives: To study the suitability of patch testing to cobalt 1.0% vs 0.5% and to analyze the co‐occurrence of allergy to cobalt, chromium, and nickel. Results: Contact allergy to cobalt was shown in 90 patients (6.6%). Eighty (5.9%) patients tested positive to cobalt 1.0%. Thirty‐seven of the 90 patients (41.1%) with cobalt allergy were missed by cobalt 0.5% and 10 (0.7%) were missed by cobalt 1.0% (P < .001). No case of patch test sensitization was reported. Allergy to chromium was seen in 2.6% and allergy to nickel in 13.3%. Solitary allergy to cobalt without nickel allergy was shown in 61.1% of cobalt‐positive individuals. Female patients had larger proportions of positive reactions to cobalt (P = .036) and nickel (P < .001) than males. Conclusion: The results speak in favor of replacing cobalt chloride 0.5% with cobalt chloride 1.0% pet. in the Swedish baseline series, which will be done 2021.