Cigarette-Smoking Intensity and Interferon-Gamma Release Assay Conversion among Persons Who Inject Drugs: A Cohort Study

Abstract

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 ≥0.7 IU/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.

Figure 1

Median IFN-γ and interquartile range at 18 months by quartiles of number of cigarettes smoked. IFN-γ > 10 IU/mL were set to 10 IU/mL due to imprecision at high concentration levels. The dotted line represents the 0.70 IU/mL cutoff which was used to define QFT conversion. IFN-γ: interferon-gamma. QFT: QuantiFERON-TB Gold In-Tube.

Figure 2

Adjusted risk ratios for QFT conversion (IFN-γ ≥ 0.70 IU/mL) at 18 months based on the final multivariable Poisson regression model with robust variance which included the following covariates: quartiles of the number of cigarettes smoked daily, education, age, gender, and alcohol use. IFN-γ: interferon-gamma. QFT: QuantiFERON-TB Gold In-Tube.