In the emergency department, how should I assess severity and select empiric antibiotics for a patient with community‑acquired pneumonia, including admission criteria and regimens for typical, atypical, comorbid, MRSA‑risk, and healthcare‑associated scenarios?

Treatment of Community-Acquired Pneumonia in the Emergency Department

In the ED, immediately assess severity using CURB-65 or PSI scoring, obtain pulse oximetry and oxygen assessment as the cornerstone first step, then initiate empiric antibiotics within the first hour targeting typical and atypical pathogens based on severity and risk factors. 1, 2

Immediate Severity Assessment

Use CURB-65 scoring to stratify risk and guide disposition decisions: 1

• Confusion (new onset)
• Urea >7 mmol/L (BUN >19 mg/dL)
• Respiratory rate ≥30 breaths/min
• Blood pressure (systolic <90 mmHg or diastolic ≤60 mmHg)
• Age ≥65 years

Disposition based on CURB-65: 1

• Score 0-1: Outpatient treatment
• Score 2: Consider hospitalization (hospital ward)
• Score ≥3: Hospital admission, consider ICU if score 4-5

Pulse oximetry and oxygen assessment must be performed immediately as the first step of treatment—this is the cornerstone of ED management. 2

Risk Stratification for Pathogen Coverage

Assess for drug-resistant pneumococcal risk factors: 1

• Age >65 years
• β-lactam therapy within 3 months
• Alcoholism
• Immunosuppressive illness
• Multiple medical comorbidities

Assess for Pseudomonas aeruginosa risk factors: 1

• Structural lung disease (bronchiectasis)
• Corticosteroid therapy (>10 mg prednisone daily)
• Broad-spectrum antibiotic use >7 days in past month
• Malnutrition

Assess for MRSA risk factors: 2, 3

• Prior MRSA infection
• Recent hospitalization
• IV drug use
• End-stage renal disease

Empiric Antibiotic Regimens by Severity

Outpatient Treatment (CURB-65 0-1, No Comorbidities)

First-line monotherapy: 1, 3

• Amoxicillin 1g three times daily, OR
• Doxycycline 100mg twice daily, OR
• Macrolide (azithromycin 500mg day 1, then 250mg daily) only if local pneumococcal macrolide resistance <25%

Outpatient with Comorbidities or CURB-65 Score 2

Combination therapy required: 1, 3

• Amoxicillin/clavulanate 875/125mg twice daily PLUS macrolide (azithromycin preferred), OR
• Respiratory fluoroquinolone monotherapy (levofloxacin 750mg daily OR moxifloxacin 400mg daily)

Hospitalized Non-Severe (Ward Admission, CURB-65 2)

Standard regimen—β-lactam PLUS macrolide combination: 1, 2

• Ceftriaxone 1-2g IV daily PLUS azithromycin 500mg IV/PO daily, OR
• Cefotaxime 1-2g IV every 8 hours PLUS azithromycin 500mg IV/PO daily

Alternative monotherapy: 1, 3

• Levofloxacin 750mg IV daily, OR
• Moxifloxacin 400mg IV daily

Severe CAP (ICU Admission, CURB-65 ≥3)

Without Pseudomonas risk factors: 1, 2, 4

• Ceftriaxone 2g IV daily PLUS azithromycin 500mg IV daily, OR
• Cefotaxime 2g IV every 8 hours PLUS azithromycin 500mg IV daily, OR
• Moxifloxacin 400mg IV daily PLUS ceftriaxone 2g IV daily

With Pseudomonas risk factors: 1, 4

• Antipseudomonal β-lactam (piperacillin-tazobactam 4.5g IV every 6 hours OR cefepime 2g IV every 8 hours OR meropenem 1g IV every 8 hours) PLUS
• Azithromycin 500mg IV daily OR respiratory fluoroquinolone (levofloxacin 750mg IV daily)

With MRSA risk factors—add to above regimens: 2, 3

• Vancomycin 15-20mg/kg IV every 8-12 hours (target trough 15-20 mcg/mL), OR
• Linezolid 600mg IV every 12 hours

Critical Diagnostic Testing in the ED

Obtain before antibiotic administration: 1, 2

• Blood cultures (two sets from separate sites) in all hospitalized patients
• Sputum Gram stain and culture if purulent sample available and processed timely
• Urine Legionella antigen in severe CAP or epidemiologic suspicion (cruise ships, hotels, recent travel)
• Urine pneumococcal antigen in hospitalized patients
• COVID-19 and influenza testing when community prevalence warrants

Procalcitonin measurement is NOT recommended for diagnosis or initial management decisions. 3

Timing of Antibiotic Administration

Initiate antibiotics immediately after diagnosis—within the first hour of ED presentation for hospitalized patients. 1, 2 Delaying antibiotic administration is associated with decreased survival in severe CAP. 5

Adjunctive Therapies in Severe CAP

Oxygen therapy: 5

• Target SpO₂ >92%
• Consider high-flow nasal oxygen or non-invasive ventilation for respiratory distress (unless severe hypoxemia present)

Fluid resuscitation: 5

• Assess for volume depletion and provide IV fluids as needed

Corticosteroids in severe CAP: 2, 4

• Systemic corticosteroid administration within 24 hours may reduce 28-day mortality in severe CAP
• Consider methylprednisolone 0.5mg/kg IV every 12 hours for 5 days in patients with severe sepsis or septic shock

Venous thromboembolism prophylaxis: 1

• Low molecular weight heparin indicated in patients with acute respiratory failure

Common Pitfalls to Avoid

Do NOT use monotherapy in severe CAP—combination therapy (β-lactam PLUS macrolide or fluoroquinolone) improves outcomes and mortality. 5, 2

Do NOT rely on sputum Gram stain alone to guide initial therapy—empiric coverage based on severity and risk factors is superior. 1

Do NOT delay antibiotics waiting for diagnostic test results—empiric therapy must be initiated immediately. 1

Do NOT use macrolide monotherapy in areas with pneumococcal macrolide resistance ≥25% or in patients with comorbidities. 1, 3

Do NOT fail to cover atypical pathogens (Legionella, Mycoplasma, Chlamydophila)—these account for significant CAP cases and require macrolide or fluoroquinolone coverage. 1

Do NOT prescribe antibiotics targeting MRSA or Pseudomonas without specific risk factors—this promotes resistance without improving outcomes. 2, 3

Treatment Duration

Minimum 3 days for hospitalized patients responding to therapy; typical duration 7-8 days for uncomplicated CAP. 1, 2 Legionella requires at least 14 days of treatment. 1

Criteria for IV to Oral Switch

Switch to oral antibiotics when: 1

• Hemodynamically stable
• Improving clinically
• Able to take oral medications
• Normal gastrointestinal absorption

Use the same antibiotic class when switching (sequential therapy). 1