The risk of COPD was higher among smokers with shorter TTFC (Table 2). Compared to smokers with longer TTFC (>60 minutes), the multivariate adjusted ORs (95% CI) for COPD were 1.48 (1.15–1.91), 1.64 (1.29–2.08), 2.18 (1.65–2.87) for those with TTFC 31–60 minutes, 6–30 minutes, and ≤5 minutes, respectively (P-trend <0.0001). Subgroup analyses by gender showed similar trends (Table 2). In gender-stratified analyses, ORs between TTFC and COPD, chronic bronchitis, and emphysema were all statistically significant in both men and women. The point estimate for the risk of COPD conferred by shortened TTFC was higher among females, although the interaction was not statistically significant (P-value = 0.13 for TTFC*gender interaction term). Importantly, the trend of increased risk of COPD with shortened TTFC was consistent by gender and disease subtype.
