Patient characteristics at baseline
| Non-ARDS patients | ARDS patients | p-value | |
| Subjects | 307 | 125 | |
| Age years | 65.8±14.4 | 63.1±17.3 | 0.26 |
| Male | 212 (69) | 75 (60) | 0.071 |
| Current smoker | 93 (31) | 34/122 (28) | 0.53 |
| Current alcohol abuse | 66 (22) | 34 (28) | 0.21 |
| Previous antibiotics# | 46 (17) | 29 (25) | 0.052 |
| Influenza vaccine | 66 (38) | 26 (34) | 0.62 |
| Pneumococcal vaccine | 33 (18) | 7 (9) | 0.054 |
| Inhaled corticosteroids | 80 (27) | 18 (15) | 0.008 |
| Systemic corticosteroids | 13 (6) | 8 (8) | 0.41 |
| Previous episode of pneumonia | 29 (11) | 7 (6) | 0.18 |
| Nursing home resident | 16 (5) | 5 (4) | 0.62 |
| Comorbidity¶ | 244 (81) | 83 (66) | 0.002 |
| Chronic respiratory disease | 152 (51) | 45 (37) | 0.009 |
| COPD | 105 (36) | 26 (22) | 0.005 |
| Asthma | 9 (3) | 4 (3) | 0.89 |
| Bronchiectasis | 8 (3) | 2 (2) | 0.73 |
| Other+ | 30 (10) | 13 (11) | 0.85 |
| Chronic cardiovascular disease | 52 (17) | 20 (16) | 0.76 |
| Diabetes mellitus | 75 (25) | 26 (21) | 0.38 |
| Neurological disease | 46 (16) | 20 (17) | 0.84 |
| Chronic renal disease | 27 (9) | 8 (6) | 0.37 |
| Chronic liver disease | 26 (9) | 12 (10) | 0.74 |
Data are presented as n, mean±sd or n (%), unless otherwise stated. ARDS: acute respiratory distress syndrome; COPD: chronic obstructive pulmonary disease. Percentages were calculated for nonmissing data. #: information on previous antibiotics was obtained in 67 (89%) cases without any significant differences in the type of antibiotics each group had received: β-lactams (n=38), fluoroquinolones (n=12), macrolides (n=9) and other (n=8); ¶: some patients may have more than one comorbid condition; +: other respiratory diseases included the sequelae of pulmonary tuberculosis, pulmonary hypertension and interstitial lung disease.