What is the recommended severity assessment and antibiotic regimen for adult community‑acquired pneumonia, including outpatient treatment for healthy patients, outpatient treatment for patients with comorbidities, inpatient ward therapy, ICU therapy, and supportive care?

Community-Acquired Pneumonia: Severity Assessment and Antibiotic Management

Severity Assessment and Site-of-Care Decision

Use validated severity scores (PSI or CURB-65) combined with clinical judgment to determine hospitalization need. 1

CURB-65 Scoring System

• Score 1 point for each: Confusion, Urea >7 mmol/L (BUN >20 mg/dL), Respiratory rate ≥30/min, Blood pressure (systolic <90 or diastolic ≤60 mmHg), Age ≥65 years 1
• CURB-65 score 0-1: Outpatient management appropriate 1
• CURB-65 score ≥2: Hospitalization warranted 12
• CURB-65 demonstrates AUC of 0.835 for predicting 30-day mortality 3

Pneumonia Severity Index (PSI)

• PSI class I-III: Suitable for outpatient care unless unstable comorbidities present 1
• PSI class IV: Consider hospitalization 1
• PSI class V: Strong indication for inpatient admission 1
• PSI shows AUC of 0.801 for mortality prediction 4

ICU Admission Criteria

Direct ICU admission required when ANY major criterion present OR ≥3 minor criteria met. 1

Major criteria:

• Septic shock requiring vasopressors 1
• Acute respiratory failure requiring intubation/mechanical ventilation 1

Minor criteria:

• Respiratory rate ≥30/min 1
• PaO₂/FiO₂ <250 1
• Multilobar infiltrates 1
• Confusion 1
• Uremia (BUN ≥20 mg/dL) 1
• Leukopenia (WBC <4,000/μL) 1
• Thrombocytopenia (platelets <100,000/μL) 1
• Hypothermia (core temperature <36°C) 1
• Hypotension requiring aggressive fluid resuscitation 1

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Outpatient Treatment for Healthy Patients (No Comorbidities)

First-line: Amoxicillin 1 g orally three times daily for 5-7 days (strong recommendation, moderate-quality evidence). 256

• Amoxicillin retains activity against 90-95% of Streptococcus pneumoniae isolates, including many penicillin-resistant strains 25
• S. pneumoniae accounts for 48% of identified CAP cases 67

Alternative: Doxycycline 100 mg orally twice daily for 5-7 days (conditional recommendation, low-quality evidence). 256

Macrolides (azithromycin 500 mg day 1, then 250 mg daily; or clarithromycin 500 mg twice daily):

• Use ONLY when local pneumococcal macrolide resistance <25% 125
• In most U.S. regions, resistance is 20-30%, making macrolide monotherapy unsafe 25

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Outpatient Treatment for Patients with Comorbidities

Comorbidities include: COPD, diabetes, chronic heart/liver/renal disease, malignancy, asplenia, immunosuppression, or antibiotic use within past 90 days. 12

Option 1: Combination Therapy (Preferred)

Amoxicillin-clavulanate 875/125 mg orally twice daily PLUS azithromycin 500 mg day 1, then 250 mg daily for 5-7 days total (strong recommendation, moderate-quality evidence). 256

• Achieves 91.5% favorable clinical outcomes 25
• Alternative β-lactams: cefpodoxime or cefuroxime (must combine with macrolide or doxycycline) 12

Option 2: Respiratory Fluoroquinolone Monotherapy

Levofloxacin 750 mg orally once daily OR moxifloxacin 400 mg orally once daily for 5-7 days (strong recommendation, moderate-quality evidence). 125

• Reserve for β-lactam allergy or when combination therapy contraindicated 25
• Active against >98% of S. pneumoniae including penicillin-resistant strains 2
• FDA warnings about serious adverse events (tendon rupture, peripheral neuropathy, aortic dissection) limit first-line use 28

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Inpatient Ward Therapy (Non-ICU)

Two equally effective regimens with strong recommendations and high-quality evidence: 12

Option 1: β-lactam + Macrolide Combination (Preferred)

Ceftriaxone 1-2 g IV once daily PLUS azithromycin 500 mg IV or orally daily 127

• Alternative β-lactams: cefotaxime 1-2 g IV every 8 hours OR ampicillin-sulbactam 3 g IV every 6 hours 12
• Covers typical pathogens (S. pneumoniae, H. influenzae, M. catarrhalis) and atypical organisms (Mycoplasma, Chlamydophila, Legionella) 12

Option 2: Respiratory Fluoroquinolone Monotherapy

Levofloxacin 750 mg IV daily OR moxifloxacin 400 mg IV daily 12

• Preferred for penicillin-allergic patients 12
• Systematic reviews show fewer clinical failures versus β-lactam/macrolide combinations 2

Critical Timing

Administer first antibiotic dose in emergency department immediately upon diagnosis; delays >8 hours increase 30-day mortality by 20-30%. 127

Diagnostic Testing

Obtain blood cultures and sputum Gram stain/culture BEFORE first antibiotic dose in ALL hospitalized patients. 12

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ICU Therapy (Severe CAP)

Combination therapy is MANDATORY for all ICU patients; β-lactam monotherapy linked to higher mortality. 128

Standard ICU Regimen

Ceftriaxone 2 g IV once 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) (strong recommendation). 128

Penicillin-Allergic ICU Patients

Aztreonam 2 g IV every 8 hours PLUS respiratory fluoroquinolone 12

Special Pathogen Coverage (Add ONLY When Risk Factors Present)

Pseudomonas aeruginosa Coverage

Risk factors: Structural lung disease (bronchiectasis, cystic fibrosis), recent hospitalization with IV antibiotics ≤90 days, prior P. aeruginosa isolation, chronic broad-spectrum antibiotic exposure ≥7 days in past month. 12

Regimen: Antipseudomonal β-lactam (piperacillin-tazobactam 4.5 g IV every 6 hours OR cefepime 2 g IV every 8 hours OR carbapenem) PLUS antipseudomonal fluoroquinolone (ciprofloxacin 400 mg IV every 8 hours OR levofloxacin 750 mg IV daily) PLUS aminoglycoside (gentamicin or tobramycin 5-7 mg/kg IV daily). 12

MRSA Coverage

Risk factors: Prior MRSA infection/colonization, recent hospitalization with IV antibiotics, post-influenza pneumonia, cavitary infiltrates on imaging. 12

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. 12

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Duration of Therapy and Transition to Oral

Minimum Duration

Treat for minimum 5 days AND until patient afebrile for 48-72 hours with no more than one sign of clinical instability (strong recommendation, Level I evidence). 127

Clinical stability criteria:

• Temperature ≤37.8°C 12
• Heart rate ≤100 bpm 12
• Respiratory rate ≤24 breaths/min 12
• Systolic blood pressure ≥90 mmHg 12
• Oxygen saturation ≥90% on room air 12
• Ability to maintain oral intake 12
• Normal mental status 12

Typical Duration

5-7 days for uncomplicated CAP 1257

Extended Duration (14-21 days)

Required ONLY for: Legionella pneumophila, Staphylococcus aureus, or Gram-negative enteric bacilli. 12

IV to Oral Transition

Switch when patient meets ALL stability criteria above—typically by hospital day 2-3. 12

Oral step-down options:

• Amoxicillin 1 g three times daily plus azithromycin 500 mg daily 2
• Continue azithromycin alone after 2-3 days IV therapy 2

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Supportive Care

Oxygen Therapy

Target PaO₂ >8 kPa (60 mmHg) and SaO₂ >92%. 1

• High-flow oxygen safe in uncomplicated pneumonia 1
• For COPD patients with ventilatory failure, guide oxygen by repeated arterial blood gases to avoid CO₂ retention 1

Monitoring Parameters

Assess temperature, respiratory rate, pulse, blood pressure, mental status, oxygen saturation at least twice daily in hospitalized patients. 11

Fluid Management

Evaluate volume depletion and consider IV fluids as needed. 1

Corticosteroids

Systemic corticosteroid administration within 24 hours of severe CAP development may reduce 28-day mortality. 79

• Administration within 36 hours decreases risk of ARDS and treatment length 9

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Treatment Failure Recognition

If no clinical improvement by day 2-3, obtain:

• Repeat chest radiograph 11
• CRP and white blood cell count 11
• Additional microbiologic specimens 11
• Consider chest CT to evaluate for complications (pleural effusion, empyema, lung abscess) 11

Escalation strategies:

• Non-severe pneumonia on amoxicillin monotherapy: Add or substitute macrolide 11
• Non-severe pneumonia on combination therapy: Switch to respiratory fluoroquinolone 11
• Severe pneumonia not responding: Consider adding rifampicin 11

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Follow-Up

Outpatients

Clinical review at 48 hours or sooner if clinically indicated. 11

All Patients

Scheduled clinical review at 6 weeks; chest radiograph ONLY for:

• Persistent symptoms 11
• Physical signs present 11
• High risk for underlying malignancy (smokers >50 years) 11

Chest radiograph NOT required before hospital discharge in patients with satisfactory clinical recovery. 11

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Critical Pitfalls to Avoid

• Never use macrolide monotherapy in hospitalized patients—fails to cover typical pathogens like S. pneumoniae 12
• Never use macrolide monotherapy when local resistance ≥25%—leads to treatment failure and breakthrough bacteremia 125
• Never delay first antibiotic dose >8 hours—increases mortality by 20-30% 127
• Never add broad-spectrum antipseudomonal or MRSA agents without documented risk factors—promotes resistance without benefit 12
• Never extend therapy beyond 7-8 days in responding patients without specific indications—increases resistance risk and C. difficile infection 12
• Never use β-lactam monotherapy in ICU patients—associated with higher mortality 128
• Never use oral cephalosporins as first-line agents—inferior pneumococcal coverage versus high-dose amoxicillin or IV ceftriaxone 2
• If patient used antibiotics within past 90 days, select agent from different class—reduces resistance risk 125