| Characteristic | All patients | PPM positive | PPM negative | p-value# |
| Subjects n | 4089 | 1898 | 2191 | |
| Male sex % | 55.0 | 61.0 | 49.8 | <0.001 |
| Age yrs | 56±15.3 | 55.3±15.5 | 57.2±15.0 | <0.001 |
| Exacerbations in previous year¶ | 2.5±1.9 | 2.4±1.6 | 2.5±2.1 | 0.614 |
| Median (IQR) duration of exacerbation before study entry days | 2 (5) | 2 (5) | 2 (6) | 0.130 |
| Past or present smoker¶ | 3280 (80.2) | 1629 (85.8) | 1651 (75.4) | <0.001 |
| Use of long-term bronchodilators or steroids | 261 (6.4) | 116 (6.1) | 147 (6.7) | 0.135 |
| Median (IQR) CRP level¶ mg·L−1 | 0.70 (1.79) | 0.69 (2.17) | 0.71 (1.56) | 0.551 |
| Fever present | 744 (18.2) | 294 (15.5) | 389 (20.5) | 0.005 |
Data are presented as mean±sd or n (%), unless otherwise stated. PPM: potentially pathogenic microorganism; IQR: interquartile range; CRP: C-reactive protein. #: PPM positive versus PPM negative; ¶: data were not collected for all patients in the study for these characteristics. Cochrane–Mantel–Haenszel test, adjusting for study, categorical variables, and using an ANOVA, also adjusting for study, for the continuous variables.