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.