How should I manage an otherwise healthy adult with community‑acquired pneumonia, including outpatient antibiotic regimen and criteria for hospital admission?

Management of Community-Acquired Pneumonia in Otherwise Healthy Adults

Outpatient Antibiotic Regimen

For previously healthy adults without comorbidities, prescribe amoxicillin 1 g orally three times daily for 5–7 days as first-line therapy. 1

• Amoxicillin retains activity against 90–95% of Streptococcus pneumoniae isolates, including many penicillin-resistant strains, making it the most effective oral agent for the predominant bacterial pathogen in CAP. 1
• Doxycycline 100 mg orally twice daily for 5–7 days is an acceptable alternative when amoxicillin is contraindicated or not tolerated. 1
• Avoid macrolide monotherapy (azithromycin or clarithromycin) in most U.S. regions where pneumococcal macrolide resistance is 20–30%, exceeding the 25% safety threshold. 1

For Patients with Comorbidities

• Adults with chronic heart, lung, liver, or renal disease, diabetes, alcoholism, malignancy, or recent antibiotic use (within 90 days) require combination therapy: amoxicillin-clavulanate 875/125 mg orally twice daily plus azithromycin (500 mg day 1, then 250 mg daily for days 2–5). 1
• Respiratory fluoroquinolone monotherapy (levofloxacin 750 mg daily or moxifloxacin 400 mg daily) is reserved for patients with β-lactam allergy or contraindications to macrolides, given FDA warnings about tendon rupture, peripheral neuropathy, and aortic dissection. 1

Treatment Duration and Monitoring

• 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. 1
• Arrange clinical review at 48 hours (or sooner if symptoms worsen) to assess response, oral intake, and adherence. 1
• If amoxicillin monotherapy fails by day 2–3, add or substitute a macrolide to cover atypical pathogens (Mycoplasma, Chlamydophila, Legionella). 1

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Hospital Admission Criteria

Hospitalize patients with a CURB-65 score ≥ 2 or Pneumonia Severity Index (PSI) class IV–V. 1

CURB-65 Components (1 point each)

• Confusion (new-onset altered mental status)
• Urea > 7 mmol/L (BUN > 20 mg/dL)
• Respiratory rate ≥ 30 breaths/min
• Blood pressure: systolic < 90 mmHg or diastolic ≤ 60 mmHg
• Age ≥ 65 years 1

Additional Absolute Indications for Admission

• Oxygen saturation < 92% on room air or PaO₂ < 60 mmHg 1
• Multilobar infiltrates on chest imaging 1
• Inability to maintain oral intake 1
• Unstable comorbid conditions 1
• Lack of adequate outpatient support or safe home environment 1

ICU Admission Criteria

Admit to ICU when any one major criterion OR ≥ 3 minor criteria are present. 1

Major criteria:

• Septic shock requiring vasopressors 1
• Respiratory failure requiring mechanical ventilation 1

Minor criteria:

• Confusion 1
• Respiratory rate ≥ 30 breaths/min 1
• Systolic blood pressure < 90 mmHg 1
• Multilobar infiltrates 1
• PaO₂/FiO₂ < 250 1

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Inpatient Antibiotic Regimen

Non-ICU Hospitalized Patients

Administer ceftriaxone 1–2 g IV once daily plus azithromycin 500 mg IV or orally daily as the standard regimen. 1

• This combination provides coverage for typical pathogens (S. pneumoniae, H. influenzae, M. catarrhalis) and atypical organisms (Mycoplasma, Chlamydophila, Legionella). 1
• Alternative β-lactams include cefotaxime 1–2 g IV every 8 hours or ampicillin-sulbactam 3 g IV every 6 hours, each combined with azithromycin. 1
• Respiratory fluoroquinolone monotherapy (levofloxacin 750 mg IV daily or moxifloxacin 400 mg IV daily) is reserved for penicillin-allergic patients. 1

ICU Patients (Severe CAP)

Escalate to ceftriaxone 2 g IV once daily plus azithromycin 500 mg IV daily for all ICU admissions; combination therapy is mandatory. 1

• β-lactam monotherapy is linked to significantly higher mortality in critically ill patients with bacteremic pneumococcal pneumonia. 1
• For penicillin-allergic ICU patients, use aztreonam 2 g IV every 8 hours plus a respiratory fluoroquinolone (levofloxacin 750 mg IV daily or moxifloxacin 400 mg IV daily). 1

Critical Timing

Administer the first antibiotic dose in the emergency department immediately upon diagnosis; delays beyond 8 hours increase 30-day mortality by 20–30%. 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) 1
• Recent hospitalization with IV antibiotics within 90 days 1
• Prior respiratory isolation of P. aeruginosa 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 or tobramycin 5–7 mg/kg IV daily). 1

MRSA

Add MRSA coverage only when risk factors are present:

• Prior MRSA infection or colonization 1
• Recent hospitalization with IV antibiotics 1
• Post-influenza pneumonia 1
• 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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Transition to Oral Therapy and Duration

Switch from IV to oral antibiotics when all clinical stability criteria are met, typically by hospital day 2–3: 1

• Temperature ≤ 37.8°C for 48–72 hours
• Heart rate ≤ 100 bpm
• Respiratory rate ≤ 24 breaths/min
• Systolic blood pressure ≥ 90 mmHg
• Oxygen saturation ≥ 90% on room air
• Able to take oral medication
• Normal mental status

Oral step-down options: Amoxicillin 1 g three times daily plus azithromycin 500 mg daily (or azithromycin alone after 2–3 days of IV therapy). 1

Total treatment duration: Minimum 5 days and until afebrile for 48–72 hours with no more than one sign of clinical instability; typical course for uncomplicated CAP is 5–7 days. 1

Extended courses (14–21 days) are required only for: Legionella pneumophila, Staphylococcus aureus, or Gram-negative enteric bacilli. 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
• Do not add broad-spectrum antipseudomonal or MRSA agents routinely—restrict to patients with documented risk factors to prevent resistance and adverse effects. 1
• Obtain blood and sputum cultures before starting antibiotics in all hospitalized patients to enable pathogen-directed therapy. 1
• Do not delay antibiotic administration while awaiting imaging or cultures; specimens should be collected rapidly, but therapy must start immediately. 1