Inverse relationship between contact allergy and psoriasis: results from a patient- and a population-based study

BACKGROUND: An inverse association between contact allergy and autoimmune diseases has been suggested. Psoriasis is an autoimmune disease and it has been debated whether contact allergy is less prevalent among patients with psoriasis. Previous studies have shown conflicting results. OBJECTIVES: To examine a possible association between contact allergy and psoriasis in two conceptually different epidemiological studies. PATIENTS AND METHODS: Two study populations were included: (i) a clinic-based register linkage study population, achieved by record linking information from the Danish National Hospital Registry identifying patients with psoriasis with information on contact allergy from a comprehensive patch test database of 15,641 patients; and (ii) a population-based cross-sectional study population organized in 1990, 1998 and 2006 and obtained by random samples from the Danish Central Personal Register. Information was obtained by questionnaire and patch testing of 4989 subjects. RESULTS: An inverse association was found between a psoriasis diagnosis and a positive patch test in both studies. The odds ratio for a person with a psoriasis diagnosis of having a positive patch test was, adjusted for sex and age, 0.58 [95% confidence interval (CI) 0.49-0.68] and 0.64 (95% CI 0.42-0.98), respectively, in the two studies. CONCLUSIONS: The finding of an inverse association between psoriasis and contact allergy may express opposite immunological mechanisms and calls for additional research in this field.