What is the recommended management and antibiotic regimen for community‑acquired pneumonia, including severity assessment (CURB‑65 or Pneumonia Severity Index), outpatient versus inpatient treatment, and specific choices for patients with recent antibiotic use, comorbidities, β‑lactam allergy, or risk factors for MRSA or Pseudomonas?

Management of Community-Acquired Pneumonia in Adults

For outpatients without comorbidities, amoxicillin 1 g orally three times daily for 5–7 days is the preferred first-line therapy, providing superior pneumococcal coverage compared with macrolides or oral cephalosporins. 1

Severity Assessment and Site-of-Care Decision

Hospital Admission Criteria

• Use CURB-65 (Confusion, Urea >7 mmol/L, Respiratory rate ≥30/min, Blood pressure <90/60 mmHg, age ≥65 years) or PSI to guide admission decisions; patients with CURB-65 ≥2 require hospitalization or intensive home health services. 2, 3
• CURB-65 is associated with lower 30-day mortality (8.6% vs 9.7%) and is more user-friendly than PSI in emergency department settings. 3, 4
• PSI classes I–III (mortality risk ≤3%) can be treated outpatient; PSI classes IV–V (mortality risk 8–35%) require hospitalization. 2
• Objective scores must be supplemented with clinical judgment regarding ability to take oral medications safely and availability of outpatient support. 2

ICU Admission Criteria

• Direct ICU admission is required for septic shock requiring vasopressors or acute respiratory failure requiring mechanical ventilation. 2
• Admit to ICU or high-level monitoring when ≥3 minor criteria are present: confusion, respiratory rate ≥30/min, systolic BP <90 mmHg, multilobar infiltrates, or PaO₂/FiO₂ <250. 2, 1

Empiric Antibiotic Regimens

Outpatient Treatment – Previously Healthy Adults

• First-line: Amoxicillin 1 g orally three times daily for 5–7 days covers 90–95% of S. pneumoniae isolates including many penicillin-resistant strains. 1
• Alternative: Doxycycline 100 mg orally twice daily for 5–7 days. 1
• Macrolides (azithromycin 500 mg day 1, then 250 mg daily; or clarithromycin 500 mg twice daily) should only be used when local pneumococcal macrolide resistance is documented <25%. 2, 1

Outpatient Treatment – Patients with Comorbidities

Comorbidities include COPD, diabetes, chronic heart/liver/renal disease, malignancy, or antibiotic use within 90 days. 1

• Option 1 – Combination therapy: β-lactam (amoxicillin-clavulanate 875/125 mg twice daily, cefpodoxime, or cefuroxime) PLUS macrolide (azithromycin or clarithromycin) OR doxycycline 100 mg twice daily. 2, 1
• Option 2 – Respiratory fluoroquinolone monotherapy: Levofloxacin 750 mg daily OR moxifloxacin 400 mg daily for 5–7 days. 2, 1

Hospitalized Patients (Non-ICU)

• Preferred regimen: Ceftriaxone 1–2 g IV daily PLUS azithromycin 500 mg IV or oral daily. 2, 1, 5
• Alternative: Respiratory fluoroquinolone monotherapy (levofloxacin 750 mg IV daily OR moxifloxacin 400 mg IV daily). 2, 1
• Alternative β-lactams include cefotaxime 1–2 g IV every 8 hours or ampicillin-sulbactam 3 g IV every 6 hours, always combined with a macrolide. 1
• For penicillin-allergic patients: Use respiratory fluoroquinolone as preferred alternative. 2, 1

Severe CAP Requiring ICU Admission

• Mandatory combination therapy: 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 respiratory fluoroquinolone (levofloxacin 750 mg IV daily or moxifloxacin 400 mg IV daily). 2, 1
• β-lactam monotherapy in ICU patients is associated with higher mortality and must be avoided. 1

Special Pathogen Coverage (Risk Factor-Based)

Pseudomonas aeruginosa Coverage

Add antipseudomonal therapy ONLY when specific risk factors are present: 2, 1

• Structural lung disease (bronchiectasis, cystic fibrosis)
• Recent hospitalization with IV antibiotics within 90 days
• Prior respiratory isolation of P. aeruginosa

Regimen: Antipseudomonal β-lactam (piperacillin-tazobactam 4.5 g IV every 6 hours, cefepime 2 g IV every 8 hours, imipenem, or meropenem) PLUS ciprofloxacin 400 mg IV every 8 hours OR levofloxacin 750 mg IV daily PLUS aminoglycoside (gentamicin or tobramycin 5–7 mg/kg IV daily). 2, 1

MRSA Coverage

Add MRSA therapy ONLY when risk factors are present: 2, 1

• Prior MRSA infection or colonization
• Recent hospitalization with IV antibiotics within 90 days
• Post-influenza pneumonia
• Cavitary infiltrates on imaging

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 base regimen. 2, 1

Duration of Therapy

• Treat for minimum of 5 days AND until patient is afebrile for 48–72 hours with no more than one sign of clinical instability. 2, 1
• Typical duration for uncomplicated CAP: 5–7 days. 2, 1, 5
• Extended duration (14–21 days) required for Legionella pneumophila, Staphylococcus aureus, or Gram-negative enteric bacilli. 2, 1

Transition from IV to Oral Therapy

Switch when patient meets ALL criteria: 2, 1

• Hemodynamically stable (SBP ≥90 mmHg, HR ≤100 bpm)
• Clinically improving
• Afebrile for 48–72 hours
• Respiratory rate ≤24 breaths/min
• Oxygen saturation ≥90% on room air
• Able to take oral medications
• Normal gastrointestinal function

Transition typically occurs by hospital day 2–3. 1

Critical Pitfalls to Avoid

• Never delay the first antibiotic dose: Administration beyond 8 hours after diagnosis increases 30-day mortality by 20–30% in hospitalized patients. 1
• Avoid macrolide monotherapy in hospitalized patients: Provides inadequate coverage for typical bacterial pathogens like S. pneumoniae. 1
• Do not use macrolide monotherapy when local resistance exceeds 25%: Most U.S. regions have 20–30% macrolide resistance among S. pneumoniae. 1
• Obtain blood and sputum cultures before antibiotics: Required in all hospitalized patients to enable pathogen-directed therapy. 2, 1
• Do not add broad-spectrum agents without documented risk factors: Restrict antipseudomonal and MRSA coverage to patients with specific risk factors to prevent resistance. 1

Diagnostic Testing

Hospitalized Patients

• Blood cultures (before antibiotics) 2
• Sputum Gram stain and culture (if able to expectorate purulent samples and no prior antibiotics) 2
• Chest radiograph 2
• Full blood count, urea, electrolytes, liver function tests 2
• C-reactive protein (when available) 2
• Oxygenation assessment 2
• Urinary antigen testing for Legionella pneumophila serogroup 1 in severe CAP 1

Outpatient Setting

• Chest radiograph confirms diagnosis and excludes complications but is not strictly required for clinically stable outpatients. 1
• Routine microbiological investigations not recommended; reserve sputum culture for non-responders or suspected tuberculosis. 1

Follow-Up and Monitoring

• Outpatients: Clinical review at 48 hours or sooner if clinically indicated. 2, 1
• All patients: Scheduled clinical review at 6 weeks; chest radiograph only for persistent symptoms, physical signs, or high risk for malignancy (smokers >50 years). 2, 1
• Treatment failure indicators (by day 2–3): Obtain repeat chest radiograph, CRP, white cell count, and additional microbiological specimens. 2, 1

Prevention

• Pneumococcal polysaccharide vaccine for all adults ≥65 years and those with high-risk conditions. 1
• Annual influenza vaccination for all patients, especially those with chronic medical illnesses. 1
• Smoking cessation counseling for all current smokers. 1