Hindawi Publishing Corporation Pulmonary Medicine Volume 2012, Article ID 828106, 7 pages doi:10.1155/2012/828106 Research Article Cigarette-Smoking Intensity and Interferon-Gamma Release Assay Conversion among Persons Who Inject Drugs: A Cohort Study Sanghyuk S. Shin, 1, 2 Manuel Gallardo, 3 Remedios Lozada, 3 Daniela Abramovitz, 2 Jose Luis Burgos, 2 Rafael Laniado-Laborin, 4 Timothy C. Rodwell, 2 Thomas E. Novotny, 1 Steffanie A. Strathdee, 2 and Richard S. Garfein 2 1 San Diego State University, 5500 Campanile Drive, San Diego, CA 92182-4162, USA 2 Division of Global Public Health, Department of Medicine, School of Medicine, University of California San Diego, 9500 Gilman Drive, MC-0507, San Diego, CA 92093-0507, USA 3 Patronato Pro-COMUSIDA, Ninos Heroes No. 697, Zona Centro, Tijuana, BC, Mexico 4 Parque Industrial Internacional, Universidad Autonoma de Baja California, Calzada Universidad 14418, Tijuana, BC, Mexico Correspondence should be addressed to Richard S. Garfein, rgarfein@ucsd.edu Received 18 September 2012; Accepted 15 November 2012 Academic Editor: Jonathan Golub Copyright © 2012 Sanghyuk S. Shin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We analyzed data from a longitudinal cohort study of persons who inject drugs (PWID) in Tijuana, Mexico, to explore whether cigarette smoking increases the risk of interferon gamma release assay (IGRA) conversion. PWID were recruited using respondent driven sampling (RDS). QuantiFERON-TB Gold In-Tube (QFT) assay conversion was defined as interferon-gamma concentrations <0.35 IU/mL at baseline and ≥0.7IU/mL at 18 months. We used multivariable Poisson regression adjusted for RDS weights to estimate risk ratios (RRs). Of 129 eligible participants, 125 (96.9%) smoked at least one cigarette during followup with a median of 11 cigarettes smoked daily, and 52 (40.3%) had QFT conversion. In bivariate analysis, QFT conversion was not associated with the number of cigarettes smoked daily (P = 0.716). Controlling for age, gender, education, and alcohol use, the RRs of QFT conversion for smoking 6–10, 11–15, and ≥16 cigarettes daily compared to smoking 0–5 cigarettes daily were 0.9 (95% confidence interval (CI), 0.5–1.6), 0.5 (95% CI, 0.3–1.2), and 0.7 (95% CI, 0.3–1.6), respectively. Although this study did not find an association between self-reported smoking intensity and QFT conversion, it was not powered sufficiently to negate such an association. Larger longitudinal studies are needed to fully explore this relationship. 1. Introduction Evidence has accumulated over the years which demonstrates a causal relationship between tobacco use and increased tuberculosis (TB) morbidity and mortality [1–6]. However, the strength of evidence for this relationship varies by TB outcome [3]. For example, while high-quality longitudinal cohort studies provide strong evidence that tobacco use increases the risk of TB disease, the evidence for the rela- tionship between tobacco use and the risk of Mycobacterium tuberculosis infection is relatively weak [3, 7, 8]. Previous studies exploring this relationship utilized cross-sectional or case-control methodologies to determine the association between “ever” or “current” smoking and lifetime infection with M. tuberculosis as determined by a single tuberculin skin test (TST) result [9–14]. Therefore, these studies were not able to assess the temporality between tobacco use and M. tuberculosis infection. For example, a participant infected with M. tuberculosis as a child who subsequently began smoking years later would contribute to the positive association between smoking and TST positivity. An improved understanding of the relationship between cigarette smoking and M. tuberculosis infection would help inform the implementation of tobacco control efforts as a