How should I evaluate and treat an otherwise healthy adult with community‑acquired pneumonia?

Evaluation and Treatment of Community-Acquired Pneumonia in Healthy Adults

Initial Diagnostic Approach

Obtain a chest radiograph to confirm the diagnosis of pneumonia when clinical features suggest infection. A new infiltrate on imaging is required to distinguish pneumonia from other respiratory conditions, particularly in patients with underlying lung disease 1. While clinical diagnosis alone may suffice in mild outpatient cases, radiographic confirmation prevents misdiagnosis and guides severity assessment 2.

Key Clinical Features to Assess

• Document fever (>38°C or <36°C), new or worsening cough, dyspnea, and purulent sputum production 3.
• Measure vital signs including respiratory rate, heart rate, blood pressure, and oxygen saturation 1.
• Assess mental status, as confusion indicates severe disease requiring hospitalization 1.
• Examine for signs of respiratory distress (tachypnea >30 breaths/min, use of accessory muscles) 4.

Microbiologic Testing Strategy

• For outpatients: Routine sputum culture and blood cultures are not recommended; reserve testing for treatment failures 5, 6.
• For hospitalized patients: Obtain two sets of blood cultures and sputum Gram stain/culture before the first antibiotic dose to enable pathogen-directed therapy 1, 3.
• Test all patients for COVID-19 and influenza when these viruses circulate in the community, as results may alter treatment and infection control measures 3, 7.
• Serologic testing and cold agglutinin measurements are not useful in initial evaluation and should not be routinely performed 5.

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Severity Assessment and Site-of-Care Decision

Use the CRB-65 score (Confusion, Respiratory rate ≥30, Blood pressure <90/60, age ≥65) to determine whether outpatient or inpatient management is appropriate. A score of 0 supports outpatient care; a score ≥2 mandates hospitalization 1, 7.

Criteria for ICU Admission

• One major criterion: Septic shock requiring vasopressors or respiratory failure requiring mechanical ventilation 1.
• Three or more minor criteria: Confusion, respiratory rate ≥30/min, systolic BP <90 mmHg, multilobar infiltrates, PaO₂/FiO₂ <250 1.

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Empiric Antibiotic Therapy

Outpatient Treatment (Previously Healthy Adults)

Prescribe amoxicillin 1 g orally three times daily for 5–7 days as first-line therapy. This regimen retains activity against 90–95% of Streptococcus pneumoniae isolates, including many penicillin-resistant strains, and provides superior pneumococcal coverage compared with oral cephalosporins 1.

• Alternative: Doxycycline 100 mg orally twice daily for 5–7 days offers coverage of typical and atypical pathogens 1, 7.
• Macrolide restriction: Use azithromycin (500 mg day 1, then 250 mg daily) or clarithromycin (500 mg twice daily) only when local pneumococcal macrolide resistance is <25%; in most U.S. regions, resistance is 20–30%, making macrolide monotherapy unsafe 1, 7.

Outpatient Treatment (Patients with Comorbidities)

For patients with COPD, diabetes, chronic heart/liver/renal disease, or recent antibiotic use, prescribe combination therapy: amoxicillin-clavulanate 875/125 mg orally twice daily plus azithromycin (500 mg day 1, then 250 mg daily) for 5–7 days 1, 7.

• Alternative: Respiratory fluoroquinolone monotherapy (levofloxacin 750 mg daily or moxifloxacin 400 mg daily) is reserved for β-lactam allergy or macrolide contraindication 1, 7.

Hospitalized Patients (Non-ICU)

Administer ceftriaxone 1–2 g IV once daily plus azithromycin 500 mg IV or orally daily. This regimen provides comprehensive coverage of typical pathogens (S. pneumoniae, H. influenzae, M. catarrhalis) and atypical organisms (Mycoplasma, Chlamydophila, Legionella) with strong evidence supporting mortality reduction 1, 3, 7.

• Alternative: Respiratory fluoroquinolone monotherapy (levofloxacin 750 mg IV daily or moxifloxacin 400 mg IV daily) is equally effective but reserved for penicillin allergy 1.
• Critical timing: Administer the first dose in the emergency department immediately; delays beyond 8 hours increase 30-day mortality by 20–30% 1.

Severe CAP Requiring ICU Admission

Combination therapy is mandatory for all ICU patients: ceftriaxone 2 g IV once daily plus azithromycin 500 mg IV daily (or a respiratory fluoroquinolone). β-lactam monotherapy is linked to higher mortality in critically ill patients 1, 3.

• For penicillin-allergic ICU patients: Use aztreonam 2 g IV every 8 hours plus a respiratory fluoroquinolone 1.

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Special Pathogen Coverage (Risk-Based Only)

Pseudomonas aeruginosa

Add antipseudomonal therapy only when specific risk factors are present: structural lung disease (bronchiectasis, cystic fibrosis), recent hospitalization with IV antibiotics within 90 days, or prior Pseudomonas isolation 1.

• 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 5–7 mg/kg IV daily) 1.

Methicillin-Resistant Staphylococcus aureus (MRSA)

Add MRSA coverage only when risk factors are present: prior MRSA infection/colonization, recent hospitalization with IV antibiotics, post-influenza pneumonia, or cavitary infiltrates on imaging 1.

• 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.

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Duration of Therapy and Transition to Oral Antibiotics

Treat for a minimum of 5 days and continue until the patient is afebrile for 48–72 hours with no more than one sign of clinical instability. Typical duration for uncomplicated CAP is 5–7 days 1, 3, 7.

• Extended courses (14–21 days) are required only for Legionella pneumophila, Staphylococcus aureus, or Gram-negative enteric bacilli 1.

Criteria for IV-to-Oral Switch

Transition to oral therapy when the patient is hemodynamically stable (systolic BP ≥90 mmHg, heart rate ≤100 bpm), clinically improving, afebrile for 48–72 hours, respiratory rate ≤24 breaths/min, oxygen saturation ≥90% on room air, and able to take oral medication—typically by hospital day 2–3 1, 3.

• Oral step-down options: Amoxicillin 1 g three times daily plus azithromycin 500 mg daily 1.

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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.
• Avoid macrolide monotherapy in outpatients when local pneumococcal macrolide resistance exceeds 25% (most U.S. regions) 1, 7.
• Do not add broad-spectrum antipseudomonal or MRSA agents routinely—restrict to patients with documented risk factors to prevent resistance and adverse effects 1.
• Do not delay antibiotic administration while awaiting imaging or cultures; specimens should be collected rapidly, but therapy must start immediately 1.
• Avoid fluoroquinolones as first-line outpatient therapy in uncomplicated cases due to FDA warnings about serious adverse events (tendon rupture, peripheral neuropathy, aortic dissection) and rising resistance 1.

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Follow-Up and Prevention

• Schedule clinical review at 48 hours (or sooner if symptoms worsen) for outpatients to assess treatment response 1.
• Arrange follow-up at 6 weeks for all hospitalized patients; obtain chest radiograph only if symptoms persist, physical signs remain, or the patient has high risk for underlying malignancy (smokers >50 years) 1, 4.
• Offer pneumococcal vaccination to all adults ≥65 years and those with high-risk conditions (20-valent conjugate vaccine alone or 15-valent conjugate vaccine followed by 23-valent polysaccharide vaccine one year later) 7.
• Recommend annual influenza vaccination for all patients 1, 7.
• Provide smoking-cessation counseling to all current smokers 1.