Appropriateness of Antibiotic Combination Therapy for Severe Community-acquired Pneumonia in South Korea

* Inclusion Criteria: * Adult patients aged 19 years or older who visited the emergency department and were hospitalized. * Findings consistent with pneumonia diagnosis, meeting all of the following criteria: * Radiologic evidence of pulmonary infiltration on chest X-ray or chest CT. * At least two or more of the following clinical signs: i) Body temperature ≥38°C or \<36°C ii) White blood cell count ≥11,000/µL or \<4,000/µL iii) Presence of purulent sputum or bronchial secretions * Within 24 hours of admission, the patient meets one major criterion or three minor criteria according to ATS/IDSA classification: * Major criteria: i) Invasive mechanical ventilation ii) Use of vasopressors (vasopressor-dependent septic shock) * Minor criteria: i) Respiratory rate ≥30 breaths/min ii) PaO2/FiO2 ≤ 250 iii) Multilobar infiltrates iv) Confusion or disorientation v) BUN ≥20 mg/dL vi) WBC \<4,000/mm³ vii) Platelet count \<100,000/mm³ viii) Hypothermia (temperature \<36°C) ix) Hypotension requiring aggressive fluid resuscitation * Exclusion Criteria: * Transferred from another hospital after \>48 hours of hospitalization * Died within 72 hours of hospital admission * Transferred to another hospital within 14 days of admission * Pneumonia occurring after \>48 hours of mechanical ventilation, including home ventilators * Detection of influenza or SARS-CoV-2 virus within 7 days of hospitalization * Identified non-pneumonia infection requiring antibiotics within 72 hours of admission * Detection of piperacillin/tazobactam-resistant Enterobacterales, Pseudomonas spp., or Acinetobacter spp. in respiratory or blood cultures within the past 90 days * Antibiotic-related adverse events observed within 72 hours of assigned treatment, requiring discontinuation or change of antibiotics