title: The association between smoking and the development of psoriatic arthritis among psoriasis patients.
abstract: OBJECTIVE
  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).
  
  
  CONCLUSIONS
  Smoking may be inversely associated with PsA among psoriasis patients. This association is not present among HLA-C*06-positive individuals.
authors.affiliation: Psoriatic Arthritis Clinic, Centre for Prognosis Studies in the Rheumatic Diseases, Toronto Western Hospital, University of Toronto, ON, Canada.
authors.forename: Lihi
authors.initials: L
authors.lastname: Eder
authors.forename: Sutha
authors.initials: S
authors.lastname: Shanmugarajah
authors.forename: Arane
authors.initials: A
authors.lastname: Thavaneswaran
authors.forename: Vinod
authors.initials: V
authors.lastname: Chandran
authors.forename: Cheryl F
authors.initials: CF
authors.lastname: Rosen
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Cook
authors.forename: Dafna D
authors.initials: DD
authors.lastname: Gladman
mesh_terms: D000328:Adult; D017677:Age Distribution; D000428:Alcohol Drinking / Q000009:adverse effects / Q000453:epidemiology; D015535:Arthritis, Psoriatic / Q000453:epidemiology* / Q000235:genetics / Q000517:prevention & control; D016022:Case-Control Studies; D005260:Female; D015236:HLA-C Antigens / Q000235:genetics; D006650:Histocompatibility Testing / Q000379:methods; D006801:Humans; D008297:Male; D008875:Middle Aged; D009864:Ontario / Q000453:epidemiology; D011565:Psoriasis / Q000453:epidemiology / Q000235:genetics; D012907:Smoking / Q000453:epidemiology* / Q000235:genetics
chemical_list: D015236:HLA-C Antigens; C528994:HLA-C*06 antigen