Microbiological aetiology by American Thoracic Society risk group
| I | II | IIIa | IIIb | IVa | IVb | All patients | |
| Nondiagnostic | 93 (73.8) | 9 (75.0) | 169 (63.3) | 169 (55.6) | 14 (38.9) | 20 (57.1) | 474 (60.8) |
| Streptococcus pneumoniae | 12 (9.5) | 2 (16.7) | 52 (19.5) | 70 (23.0) | 8 (22.2) | 6 (17.1) | 150 (19.2) |
| Atypical bacteria# | 12 (9.5) | 2 (0.7) | 13 (4.3) | 0 (0) | 1 (2.9) | 28 (6.4) | |
| Legionella | 3 (2.4) | 7 (2.6) | 6 (2.0) | 3 (8.3) | 3 (8.6) | 22 (2.8) | |
| Mixed | 3 (2.4) | 8 (3.0) | 20 (6.6) | 4 (11.1) | 4 (11.4) | 39 (5.0) | |
| Virus | 1 (0.8) | 1 (8.3) | 10 (3.7) | 16 (5.3) | 3 (8.3) | 31 (4.0) | |
| Haemophilus influenzae | 1 (0.8) | 8 (3.0) | 4 (1.3) | 3 (8.3) | 1 (2.9) | 17 (2.2) | |
| Other | 1 (0.8) | 2 (0.7) | 4 (1.3) | 1 (2.8) | 8 (1.0) | ||
| Nonfermenting | 4 (1.5) | 1 (0.3) | 5 (0.6) | ||||
| Staphylococcus aureus | 5 (1.9) | 5 (0.6) | |||||
| Enterobacteriaceae | 1 (0.3) | 1 (0.1) | |||||
| Total | 126 | 12 | 267 | 304 | 36 | 35 | 780 |
Data are presented as n (%). #: Mycoplasma pneumoniae, Chlamydophila pneumoniae and Coxiella burnetii.