Criteria for severe community-acquired pneumonia according to the 2001 American Thoracic Society Guidelines 51# (arranged according to major and minor criteria)
| Baseline (minor) criteria assessed at admission |
| Respiratory rate >30·min−1 |
| Severe respiratory failure (Pa,O2/FI,O2 ratio <250) |
| Bilateral involvement in chest radiograph |
| Involvement of more than two lobes in chest radiograph (multilobar involvement) |
| Systolic blood pressure <90 mmHg |
| Diastolic blood pressure <60 mmHg |
| Major criteria assessed at admission or during clinical course |
| Requirement for mechanical ventilation |
| Increase in the size of infiltrates by ≥50% in the presence of clinical nonresponse to treatment or deterioration (progressive infiltrates) |
| Requirement of vasopressors >4 h (septic shock) |
| Serum-creatinine ≥2 mg·dL−1 or increase of ≥2 mg·dL−1 in a patient with previous renal disease or acute renal failure requiring dialysis (acute renal failure) |
Pa,O2: arterial oxygen tension; FI,O2: inspiratory oxygen fraction. #: the presence of at least two minor criteria or one major criterion defines severe pneumonia, i.e. pneumonia requiring admission at the ICU.