[Association between smoking and tuberculosis infection]

Several reports show smoking as a risk factor of tuberculosis (TB) infection, especially in prisoners, emigrants, the homeless, or people in areas where TB is endemic. These reports mostly used the tuberculin test to detect TB. However, there is no report evaluating smoking as a risk factor of TB infection among people coming into contact with TB with the use of the Interferon-Gamma Release Assays (IGRA) test.

We compared TB infection in smokers and non-smokers who came into contact with TB infection by using the IGRA test. We retrospectively collected information about people coming into contact with TB who visited the Daiichi Dispensary from July 1, 2011 to June 30, 2012. They were divided into 2 groups (IGRA positive or negative) and smoking (present/past or never).

Out of 390 subjects who came into contact with TB examined, 229 were male and 161 were female. The mean age was 39.0 years, 98 were present smokers, 69 were past smokers, and 223 were never-smokers. There were 19 IGRA-positive and 371 IGRA-negative subjects. The IGRA positive rate was 4.9%. Out of 19 IGRA-positive subjects, 13 were smokers or ever-smoker (68.4%). Out of 371 IGRA-negative subjects, 154 cases were smoker or ever-smoker (41.5%). Smoking experience (present and past) was statistically significant in the IGRA-positive group. There were no significant differences in sex, age, drinking habits, and level of contact. Multivariate analysis showed smoking was only one independent risk factor for being IGRA-positive (odds ratio 3.06, 95% confidence interval: 1.14-8.21, p = 0.027).

Our results suggest that smoking experience in subjects coming into contact with TB is a risk factor for TB infection. TB cases in smokers are reported to be more severe and have delayed detection of disease. They are also more likely to infect those who come in contact with them. If TB source cases and their contacts are both smokers and co-exist in a narrow and limited area, the contacts might be at higher risk of exposure to TB-contaminated air than non-smokers.