Erious events (hospitalization and/or mortality). The “systemic COPD phenotype” was characterized by high prevalence of obesity and cardiovascular disease [12], corresponding to our Phenotype 3. However, the “severe respiratory phenotype” differed from our Phenotype 2 in that patients were not younger and had no malnutrition [12]. Such difference may be related to the recruitment of a specific population of subject at the time of first hospitalization for COPD exacerbation. Female represented only 6? of subjects in the Garcia-Aymerich’s 
study, whereas they represented up to one third of subjects 22948146 in our Phenotype 2. Interestingly, recent data suggested that female gender is a risk factor for early onset COPD and more severe disease at young age [18]. These phenotypic differences underline the need for external validation of identified phenotypes across multiple populations.Some limitations have to be taken into account when interpreting our results. Although repeated and severe exacerbations are important predictors of mortality [19], we had no data on exacerbations. Our study was based on the assessment of COPD patients coming to an outpatient clinic and smokers recruited for a study on lung cancer screening. Although these patients had a wide range of disease severity, they may not represent the COPD population at large and different results may be observed when studying different populations of patients. COPD subjects recruited as part of the NELSON study [13] were submitted to systematic screening and may not be representative of symptomatic subjects receiving a diagnosis of COPD. The inclusion of these subjects allowed for studying COPD subjects with a wide range of disease severity because the NELSON subjects were mostly in GOLD stage I and II, whereas the LEUVEN subjects were mostly in GOLD stage II, III and IV. Interestingly, 95 of the NELSON subjects and 19 of the LEUVEN subjects clustered in Phenotype 1, in which mortality was almost get LED 209 absent. Thus, our methodology was able to identify subjects at low risk of mortality in subjects 
with previously diagnosed and with previously undiagnosed COPD. In this real-life COPD population, 8/527 (1.5 ) subjects were lost to follow-up and the exact date of death was unavailable in 8/50 (16 ) subjects who died during follow-up. Because survival analyses were performed in 511/527 (97 ) subjects, missing dataFigure 3. Mortality distribution by GOLD stage in Phenotype 2 and 3. At the end of the follow-up period, 20/97 (20.6 ) and 29/203 (14.3 ) subjects had died in Phenotype 2 and 3, respectively. Distribution of dead subjects by GOLD stage is expressed as total number of death in each phenotype. The majority of Phenotype 2 subjects who died had very severe airflow limitation, whereas only 25 of Phenotype 3 subjects who died were in GOLD stage IV. doi:10.1371/journal.pone.order Emixustat (hydrochloride) 0051048.gFigure 4. Kaplan-Meier analysis of mortality between Phenotypes. Subjects in Phenotype 2 and 3 were at higher risk of mortality than subjects in Phenotype 1 (each comparison, P,0.0001; log-rank test). However, no significant difference was observed between Phenotype 2 and 3, indicating that during the period of observation both group had comparable mortality. doi:10.1371/journal.pone.0051048.gCOPD Phenotypes at High Risk of MortalityTable 3. Cox model analysis of mortality between phenotypes.Unadjusted Hazard Ratio [95 CI] Phenotype 2 vs. 3 Phenotype 2 vs. 1 Phenotype 3 vs. 1 CI: confidence interval. doi:10.1.Erious events (hospitalization and/or mortality). The “systemic COPD phenotype” was characterized by high prevalence of obesity and cardiovascular disease [12], corresponding to our Phenotype 3. However, the “severe respiratory phenotype” differed from our Phenotype 2 in that patients were not younger and had no malnutrition [12]. Such difference may be related to the recruitment of a specific population of subject at the time of first hospitalization for COPD exacerbation. Female represented only 6? of subjects in the Garcia-Aymerich’s study, whereas they represented up to one third of subjects 22948146 in our Phenotype 2. Interestingly, recent data suggested that female gender is a risk factor for early onset COPD and more severe disease at young age [18]. These phenotypic differences underline the need for external validation of identified phenotypes across multiple populations.Some limitations have to be taken into account when interpreting our results. Although repeated and severe exacerbations are important predictors of mortality [19], we had no data on exacerbations. Our study was based on the assessment of COPD patients coming to an outpatient clinic and smokers recruited for a study on lung cancer screening. Although these patients had a wide range of disease severity, they may not represent the COPD population at large and different results may be observed when studying different populations of patients. COPD subjects recruited as part of the NELSON study [13] were submitted to systematic screening and may not be representative of symptomatic subjects receiving a diagnosis of COPD. The inclusion of these subjects allowed for studying COPD subjects with a wide range of disease severity because the NELSON subjects were mostly in GOLD stage I and II, whereas the LEUVEN subjects were mostly in GOLD stage II, III and IV. Interestingly, 95 of the NELSON subjects and 19 of the LEUVEN subjects clustered in Phenotype 1, in which mortality was almost absent. Thus, our methodology was able to identify subjects at low risk of mortality in subjects with previously diagnosed and with previously undiagnosed COPD. In this real-life COPD population, 8/527 (1.5 ) subjects were lost to follow-up and the exact date of death was unavailable in 8/50 (16 ) subjects who died during follow-up. Because survival analyses were performed in 511/527 (97 ) subjects, missing dataFigure 3. Mortality distribution by GOLD stage in Phenotype 2 and 3. At the end of the follow-up period, 20/97 (20.6 ) and 29/203 (14.3 ) subjects had died in Phenotype 2 and 3, respectively. Distribution of dead subjects by GOLD stage is expressed as total number of death in each phenotype. The majority of Phenotype 2 subjects who died had very severe airflow limitation, whereas only 25 of Phenotype 3 subjects who died were in GOLD stage IV. doi:10.1371/journal.pone.0051048.gFigure 4. Kaplan-Meier analysis of mortality between Phenotypes. Subjects in Phenotype 2 and 3 were at higher risk of mortality than subjects in Phenotype 1 (each comparison, P,0.0001; log-rank test). However, no significant difference was observed between Phenotype 2 and 3, indicating that during the period of observation both group had comparable mortality. doi:10.1371/journal.pone.0051048.gCOPD Phenotypes at High Risk of MortalityTable 3. Cox model analysis of mortality between phenotypes.Unadjusted Hazard Ratio [95 CI] Phenotype 2 vs. 3 Phenotype 2 vs. 1 Phenotype 3 vs. 1 CI: confidence interval. doi:10.1.