Aetiology according to severity score (PSI) (number of patients with known aetiology in specific risk class)
| Aetiology | PSI I–III (n=659) | PSI IV (n=500) | PSI V (n=301) | Total (n=1460) | p Value |
| Streptococcus pneumoniae | 276 (42) | 205 (41) | 132 (44) | 613 (42) | 0.728 |
| Haemophilus influenzae | 27 (4) | 28 (6) | 15 (5) | 70 (5) | 0.488 |
| Moraxella catarrhalis | 2 (0.3) | 2 (0.4) | 1 (0.3) | 5 (0.3) | 0.961 |
| Atypical bacteria* † ‡ | 163 (25) | 77 (15) | 23 (8) | 263 (18) | <0.001 |
| Legionella pneumophila‡ | 54 (8) | 50 (10) | 14 (5) | 118 (8) | 0.027 |
| Mycoplasma pneumoniae* † | 51 (8) | 12 (2) | 2 (1) | 65 (4) | <0.001 |
| Chlamydophila pneumoniae | 31 (5) | 13 (3) | 6 (2) | 50 (3) | 0.046 |
| Coxiella burnetii* † | 27 (4) | 2 (0.4) | 1 (0.3) | 30 (2) | <0.001 |
| Virus | 62 (9) | 57 (11) | 29 (10) | 148 (10) | 0.511 |
| Staphylococcus aureus | 9 (1) | 10 (2) | 6 (2) | 25 (2) | 0.651 |
| MS | 5 (1) | 5 (1) | 4 (1) | 14 (1) | 0.697 |
| MR | 4 (1) | 5 (1) | 2 (1) | 11 (1) | 0.731 |
| GNEB† | 7 (1) | 9 (2) | 11 (4) | 27 (2) | 0.022 |
| Pseudomonas aeruginosa† ‡ | 9 (1) | 17 (3) | 23 (8) | 49 (3) | <0.001 |
| Mixed | 84 (13) | 73 (15) | 51 (17) | 208 (14) | 0.217 |
| Others | 20 (3) | 22 (4) | 10 (3) | 52 (4) | 0.448 |
Data are expressed as n (%).
PSI was not available in three patients.
↵* p<0.005, PSI classes I–III vs class IV.
↵† p<0.005, PSI classes I–III vs class V.
↵‡ p<0.005, PSI class IV vs class V.
GNEB, Gram-negative enteric bacilli; MR, methicillin-resistant; MS, methicillin-sensitive; PSI, pneumonia sensitivity index.