Variability in patch test reactions - first report from the Norwegian Patch Test Registry

Background: A nation‐wide Norwegian Patch Test Registry (NOLAR) was established in 2005 as a collaboration between six dermatology departments. International, multi‐centre studies have documented great variability in the frequency of positive patch test reactions, considered as mainly due to heterogeneity of test populations. Objectives: To analyse the variability of positive test reactions by studying patch tests performed at the six collaborating departments, using standardized procedures. Materials and methods: Data from all patch tests (n = 2089) performed in 2007–2008 as registered in the NOLAR program. Differences between centres were analysed using Exact Pearson χ2 test. Results: Between the centres, positive test reactions (+, ++, or +++) varied significantly for 8 of the 26 allergens in the European Baseline Series. When considering strong reactions (++ or +++) only, the differences were statistically significant for six of these allergens, i.e. cobalt chloride, potassium dichromate, p‐phenylenediamine, formaldehyde, paraben mix, and mercaptobenzothiazole. Conclusion: The results indicate regional differences in the prevalence of sensitization to certain allergens within the Norwegian population, although inter‐observer differences cannot be ruled out as a factor.