The association between TTFC and COPD was consistent in a sensitivity analysis limited to participants without a prior diagnosis of lung cancer (yes/no) (Table C in S1 File) and, in a separate model, after adjusting for such diagnoses (n = 33 cases; n = 44 controls). In models limited to participants with complete (non-imputed) follow-up data (Table D and Table E in S1 File), results were similar to our main findings. In models which considered emphysema and chronic bronchitis separately, adjusting for the other condition attenuated the association slightly but statistical significance was retained (Table F in S1 File). The association between TTFC and the risk of COPD in non-Hispanic whites (Table G in S1 File) was almost identical that for the overall sample of current smokers. Among African Americans, those with shorter TTFC had the greatest risk of COPD and emphysema, compared to those with longest TTFC (Table H in S1 File). Given the small number of African Americans in this sample (n = 360), risk estimates for chronic bronchitis could not be produced.
