Smoking is a potential confounder of the Chlamydia pneumoniae-coronary artery disease association.

Two recent studies, which did not adequately control for smoking status, found associations between Chlamydia pneumoniae serological titers and various manifestations of coronary artery disease (CAD). The validity of C. pneumoniae-CAD associations found in case-control studies has been criticized on the basis that smoking, known to be associated with CAD and hypothesized to be associated with C. pneumoniae seroreactivity via an increased prevalence of respiratory infection in smokers, could be an uncontrolled confounder in these studies. We investigated associations between current smoking status and C. pneumoniae serological titers in a cohort of 365 outpatients (mean age, 34 years) with respiratory illness. Current smokers were significantly (p = 0.04) more likely than nonsmokers to have C. pneumoniae titers greater than or equal to 1:128, and there was a significant (p less than 0.05) "dose-response" association between titer category and smoking, which persisted after controlling for age and sex in a logistic-regression model. These results support the hypothesis that smoking may be a confounder of the association of C. pneumoniae antibody titer and smoking-associated diseases such as CAD. Future studies into these associations should control for cigarette use.