| Characteristics | Subjects | Severe exacerbation | p-value# |
| Sex | |||
| Females | 518 | 47 (9.1) | 0.076 |
| Males | 448 | 27 (6.0) | |
| Smoking status | |||
| Never | 445 | 35 (7.9) | 0.010 |
| Former | 267 | 11 (4.1) | |
| Current | 245 | 26 (10.6) | |
| Unknown | 9 | 2 (22.2) | |
| Type of sample | |||
| Random | 504 | 32 (6.3) | 0.109 |
| Symptomatic | 462 | 42 (9.1) | |
| Country | |||
| Belgium | 31 | 2 (6.5) | 0.024 |
| Estonia | 10 | 0 (0) | |
| France | 111 | 3 (2.7) | |
| Germany | 32 | 2 (6.3) | |
| Iceland | 58 | 3 (5.2) | |
| Italy | 36 | 6 (16.7) | |
| Norway | 54 | 2 (3.7) | |
| Spain | 149 | 21 (14.1) | |
| Sweden | 275 | 24 (8.7) | |
| Switzerland | 60 | 2 (3.3) | |
| UK | 119 | 7 (5.9) | |
| USA | 31 | 2 (6.5) |
Data are presented as n or n (%), unless otherwise stated. #: p-value from Chi-squared test or Fisher's exact test.