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 chi(2) 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.