When a patient who is hospitalized/on mechanical ventilation for > 48 hourrs develops new or progressive infiltrates on chest radiography and has at least 2 of the following features:

1. Fever> 100.4 ℉.
2. Leucocytosis (>12000/µl) or leucopenia (<4000/ µl).
3. Altered mental status with no other recognizable cause in the elderly.
4. New onset purulent sputum or change in sputum character.
5. Worsening gas exchange (i.e. increased FiO2 requirement).
6. New onset or worsening cough or dyspnea.
7. Rales or bronchial breathing.