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.