Community-Acquired Pneumonia: Initial Assessment and Antibiotic Regimens
Initial Assessment and Severity Stratification
Use the Pneumonia Severity Index (PSI) or CURB-65 score to determine site of care immediately upon diagnosis. 1, 2
- PSI classes I–III: Treat outpatient unless unstable comorbidities exist 1, 2
- PSI class IV: Consider hospitalization 1, 2
- PSI class V: Strong indication for inpatient admission 1, 2
- CURB-65 ≥ 2: Hospital admission recommended 1, 2
ICU admission is mandatory when any one major criterion OR ≥3 minor criteria are present: 1, 2
- Major criteria: Septic shock requiring vasopressors OR respiratory failure requiring mechanical ventilation 1, 2
- Minor criteria: Confusion, respiratory rate ≥30/min, systolic BP <90 mmHg, multilobar infiltrates, PaO₂/FiO₂ <250 1, 2
Outpatient Treatment
Previously Healthy Adults (No Comorbidities)
Amoxicillin 1 g orally three times daily for 5–7 days is the preferred first-line therapy because it retains activity against 90–95% of Streptococcus pneumoniae isolates, including many penicillin-resistant strains. 1, 2
- Alternative: Doxycycline 100 mg orally twice daily for 5–7 days 1, 2
- Macrolides (azithromycin 500 mg day 1, then 250 mg daily; clarithromycin 500 mg twice daily) should ONLY be used when local pneumococcal macrolide resistance is documented <25%—in most U.S. regions resistance is 20–30%, making macrolide monotherapy unsafe 1, 2
Adults with Comorbidities (COPD, Diabetes, Chronic Heart/Liver/Renal Disease, Malignancy, Recent Antibiotic Use)
Combination therapy is required: 1, 2
- Option 1: Amoxicillin-clavulanate 875/125 mg orally twice daily PLUS azithromycin (500 mg day 1, then 250 mg daily days 2–5) OR doxycycline 100 mg twice daily 1, 2
- Option 2: Respiratory fluoroquinolone monotherapy—levofloxacin 750 mg daily OR moxifloxacin 400 mg daily for 5–7 days 1, 2
Critical pitfall: Fluoroquinolones should be reserved for patients with β-lactam allergy or contraindications due to FDA warnings about tendon rupture, peripheral neuropathy, and aortic dissection 1, 2
Inpatient Non-ICU Treatment
Two equally effective regimens exist with strong recommendations and high-quality evidence: 1, 2
Preferred Regimen
Ceftriaxone 1–2 g IV daily PLUS azithromycin 500 mg IV or oral daily 1, 2
- Alternative β-lactams: Cefotaxime 1–2 g IV every 8 hours OR ampicillin-sulbactam 3 g IV every 6 hours, always combined with azithromycin 1, 2
Alternative Regimen
Respiratory fluoroquinolone monotherapy: Levofloxacin 750 mg IV daily OR moxifloxacin 400 mg IV daily 1, 2
Critical timing: Administer the first antibiotic dose in the emergency department immediately—delays beyond 8 hours increase 30-day mortality by 20–30% 1, 2
Obtain blood cultures and sputum Gram stain/culture BEFORE the first antibiotic dose in all hospitalized patients to enable pathogen-directed therapy 1, 2
ICU Treatment (Severe CAP)
Combination therapy is MANDATORY for all ICU patients—β-lactam monotherapy is associated with higher mortality in critically ill patients with bacteremic pneumococcal pneumonia. 1, 2, 3
Standard ICU Regimen
Ceftriaxone 2 g IV daily (or cefotaxime 1–2 g IV every 8 hours or ampicillin-sulbactam 3 g IV every 6 hours) PLUS azithromycin 500 mg IV daily OR a respiratory fluoroquinolone (levofloxacin 750 mg IV daily or moxifloxacin 400 mg IV daily) 1, 2
Penicillin-Allergic ICU Patients
Aztreonam 2 g IV every 8 hours PLUS levofloxacin 750 mg IV daily OR azithromycin 500 mg IV daily 1, 2
Special Pathogen Coverage (Risk-Based Only)
Antipseudomonal Coverage (Add ONLY When Risk Factors Present)
Risk factors: Structural lung disease (bronchiectasis, cystic fibrosis), recent hospitalization with IV antibiotics within 90 days, prior respiratory isolation of Pseudomonas aeruginosa, chronic broad-spectrum antibiotic exposure ≥7 days in past month 1, 2
Regimen: Piperacillin-tazobactam 4.5 g IV every 6 hours PLUS ciprofloxacin 400 mg IV every 8 hours OR levofloxacin 750 mg IV daily PLUS an aminoglycoside (gentamicin or tobramycin 5–7 mg/kg IV daily) 1, 2
MRSA Coverage (Add ONLY When Risk Factors Present)
Risk factors: Prior MRSA infection/colonization, recent hospitalization with IV antibiotics, post-influenza pneumonia, cavitary infiltrates on imaging 1, 2
Regimen: Vancomycin 15 mg/kg IV every 8–12 hours (target trough 15–20 µg/mL) OR linezolid 600 mg IV every 12 hours, added to the base regimen 1, 2
Duration of Therapy and Transition to Oral
Minimum duration: 5 days AND continue until the patient is afebrile for 48–72 hours with no more than one sign of clinical instability 1, 2
- Typical duration for uncomplicated CAP: 5–7 days 1, 2
- Extended duration (14–21 days) ONLY for: Legionella pneumophila, Staphylococcus aureus, or Gram-negative enteric bacilli 1, 2
Switch from IV to oral when ALL stability criteria are met: 1, 2
- Temperature ≤37.8°C
- Heart rate ≤100 bpm
- Respiratory rate ≤24 breaths/min
- Systolic BP ≥90 mmHg
- Oxygen saturation ≥90% on room air
- Able to take oral medication
- Normal mental status
Typical transition occurs by hospital day 2–3 1, 2
Critical Pitfalls to Avoid
- NEVER use macrolide monotherapy in hospitalized patients—it fails to cover typical pathogens like S. pneumoniae and leads to treatment failure 1, 2, 3
- NEVER use macrolide monotherapy in outpatients when local pneumococcal macrolide resistance exceeds 25% (most U.S. regions) 1, 2
- NEVER delay antibiotic administration beyond 8 hours—this increases 30-day mortality by 20–30% 1, 2
- NEVER add broad-spectrum antipseudomonal or MRSA agents routinely—restrict to patients with documented risk factors to prevent resistance 1, 2
- NEVER use β-lactam monotherapy in ICU patients—combination therapy is mandatory and reduces mortality 1, 2, 3
Follow-Up and Monitoring
- Clinical review at 48 hours (or sooner if symptoms worsen) for outpatients 1, 2
- If no improvement by day 2–3: Obtain repeat chest radiograph, CRP, white cell count, and consider chest CT to evaluate for complications (pleural effusion, empyema, lung abscess) 1, 2
- Routine follow-up at 6 weeks for all patients; chest radiograph ONLY for those with persistent symptoms, physical signs, or high risk for underlying malignancy (smokers >50 years) 1, 2