Smoking in tuberculosis patients increases the risk of infection in their contacts.

OBJECTIVE: To determine the risk of latent tuberculous infection (LTBI) among contacts of smokers with tuberculosis (TB). METHODS: A study was conducted to determine the prevalence of LTBI among contacts of TB cases aged >14 years in Catalonia, Spain. A survey was carried out for each TB case and their contacts. LTBI was diagnosed using the tuberculin skin test (≥5 mm). The risk of LTBI associated with smoking was determined by multi-variate logistic regression analysis, with adjusted odds ratio (aOR) and their 95% confidence intervals (CI). RESULTS: The smoking prevalence among TB cases was 40.7% (439/1079). The prevalence of LTBI among their contacts was 29.7% (2281/7673). It was higher among contacts of smoking index cases (35.3%) than among those of non-smokers (25.7%). Smoking was independently associated with an increased risk of LTBI among contacts (aOR 1.5, 95%CI 1.3-1.7), and was estimated to be responsible for 12.8% of infections. CONCLUSIONS: Index case smoking increases the risk of LTBI and should be systematically investigated. A reduction in smoking could lower the risk of infection substantially.