Aim To investigate the association between smoking and psoriatic arthritis (PsA) among patients with psoriasis and its interaction with the HLA-C*06 allele.
Methods In this exploratory case–control study, smoking status was determined at the time of the diagnosis of arthritis for PsA patients and at their first study visit for psoriasis patients, when they were confirmed not to have PsA. The proportions of patients exposed to smoking were compared in patients with PsA to those with psoriasis alone. A logistic regression model was constructed to test the independent association of smoking and PsA after adjusting for potential confounders. The statistical interaction between HLA-C*06 and smoking was tested through a regression model.
Results The proportions of current and past smokers were higher in the psoriasis group compared with the PsA group (30.2% vs 23.4% and 26.7% vs 22.3%, p=0.001, respectively). On multivariate analysis being a current smoker versus a lifetime non-smoker remained inversely associated with PsA (OR 0.57, p=0.002), while past smoker versus lifetime non-smoker status was no longer significant. In a subgroup analysis, smoking remained inversely associated with PsA only among patients who were HLA-C*06 negative. Regression analysis revealed that the interaction between smoking status (ever smoked vs lifetime non-smoker) and HLA-C*06 was statistically significant (p=0.01).
Conclusion Smoking may be inversely associated with PsA among psoriasis patients. This association is not present among HLA-C*06-positive individuals.
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Funding The University of Toronto PsA programme is supported by a grant from the Krembil Foundation, as well as by the Arthritis Society SPARCC National Research Initiative. LE is supported by a fellowship grant from the Canadian Arthritis Network and an Abbott PsA Fellowship. VC is supported by a Canadian Institutes of Health Research—clinical research initiative fellowship and the Krembil Foundation. RJC holds a Canada research chair in statistical methods for health research.
Competing interests None.
Patient consent Obtained.
Ethics approval This study was conducted with the approval of the university health network research ethics board.
Provenance and peer review Not commissioned; externally peer reviewed.