Community-Acquired Pneumonia: Evaluation, Severity Assessment, and Antibiotic Management
Diagnostic Confirmation
Obtain a chest radiograph (posteroanterior and lateral views) to confirm the diagnosis in all patients with suspected CAP. A definitive diagnosis requires both compatible clinical features (new cough, dyspnea, fever >38°C or hypothermia <36°C, abnormal breath sounds) and radiographic evidence of a pulmonary infiltrate; neither component alone is sufficient 1, 2. Physical examination findings such as crackles or bronchial breath sounds support the diagnosis but cannot replace imaging 1. If the chest X-ray is negative but clinical suspicion remains high, obtain a chest CT scan due to its higher sensitivity, though CT-only findings have uncertain clinical significance 1.
Perform pulse oximetry on every patient to detect unsuspected hypoxemia, which influences disposition decisions and supports the diagnosis even when other signs are subtle 1.
Microbiological Testing Strategy
For otherwise healthy outpatients with mild disease, do not perform extensive microbiological testing; start empirical antibiotic therapy promptly 1, 2. Up to 50% of CAP cases have no identifiable pathogen, and testing should never delay antibiotic initiation 1, 3.
For hospitalized patients, obtain two sets of blood cultures before antibiotics (yield approximately 11%), along with sputum Gram stain and culture if a good-quality specimen is available 1. Test all patients for COVID-19 and influenza when these viruses are common in the community, as their diagnosis may affect treatment and infection prevention strategies 2.
Severity Assessment and Site-of-Care Decision
Use validated severity scores (Pneumonia Severity Index or CURB-65) combined with clinical judgment to determine hospitalization need 4. The primary decision is whether hospitalization is required, as inpatient care costs approximately $7,500 compared with $150–$350 for outpatient management 1.
Outpatient Management Criteria
- PSI classes I–III are appropriate for outpatient care unless unstable comorbidities exist 4
- CURB-65 score 0–1 supports outpatient management 4
- Absence of high-risk features (see below) 1
Hospitalization Criteria
- Age >65 years 1
- Altered mental status 1
- Systolic blood pressure <90 mmHg 1
- Hypothermia (<36°C) 1
- CURB-65 score ≥2 4
- PSI class IV–V 4
ICU Admission Criteria
Admit to ICU when one major criterion (septic shock requiring vasopressors OR respiratory failure requiring mechanical ventilation) OR ≥3 minor criteria are present 4. Minor criteria include: confusion, respiratory rate ≥30/min, systolic BP <90 mmHg, multilobar infiltrates, PaO₂/FiO₂ <250, uremia, leukopenia, thrombocytopenia, hypothermia, or need for aggressive fluid resuscitation 4.
Antibiotic Regimens
Outpatient Treatment: Previously Healthy Adults
First-line: Amoxicillin 1 g orally three times daily for 5–7 days 4. High-dose amoxicillin retains activity against 90–95% of Streptococcus pneumoniae isolates, including many penicillin-resistant strains, and provides superior pneumococcal coverage compared with oral cephalosporins 4.
Alternative: Doxycycline 100 mg orally twice daily for 5–7 days 4. This offers reliable coverage of both typical bacterial pathogens and atypical organisms (Mycoplasma, Chlamydophila, Legionella) 4.
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% 4. In most U.S. regions, macrolide resistance is 20–30%, making macrolide monotherapy unsafe as first-line therapy 4.
Outpatient Treatment: Adults with Comorbidities
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 4. This regimen yields approximately 91.5% favorable clinical outcomes by covering typical bacteria and atypical pathogens 4.
Alternative: Respiratory fluoroquinolone monotherapy (levofloxacin 750 mg daily OR moxifloxacin 400 mg daily) for 5–7 days 4. Reserve fluoroquinolones for patients with β-lactam allergy or when combination therapy is contraindicated due to FDA safety warnings (tendon rupture, peripheral neuropathy, aortic dissection) 4.
Hospitalized Non-ICU Patients
Standard regimen: Ceftriaxone 1–2 g IV once daily PLUS azithromycin 500 mg IV or orally daily 4, 2. This combination provides coverage for typical pathogens (S. pneumoniae, H. influenzae, M. catarrhalis) and atypical organisms with strong evidence and high-quality data 4.
Alternative: Respiratory fluoroquinolone monotherapy (levofloxacin 750 mg IV daily OR moxifloxacin 400 mg IV daily) 4. Systematic reviews demonstrate fewer clinical failures and treatment discontinuations compared with β-lactam/macrolide combinations 4. Use this regimen for penicillin-allergic patients 4.
Obtain blood cultures and sputum Gram stain/culture before initiating antibiotics in all hospitalized patients to enable pathogen-directed therapy and safe de-escalation 4.
ICU Patients (Severe CAP)
Mandatory combination therapy: 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) 4. β-lactam monotherapy is linked to higher mortality in critically ill patients with bacteremic pneumococcal pneumonia 4.
For penicillin-allergic ICU patients: Aztreonam 2 g IV every 8 hours PLUS respiratory fluoroquinolone 4.
Special Pathogen Coverage (Only When Risk Factors Present)
Antipseudomonal coverage: Add only for patients with structural lung disease, recent hospitalization with IV antibiotics (≤90 days), or prior Pseudomonas aeruginosa isolation 4. 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 aminoglycoside (gentamicin or tobramycin 5–7 mg/kg IV daily) 4.
MRSA coverage: Add only for patients with prior MRSA infection/colonization, recent hospitalization with IV antibiotics, post-influenza pneumonia, or cavitary infiltrates 4. 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 4.
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 4, 2. Typical duration for uncomplicated CAP is 5–7 days 4.
Extended courses (14–21 days) are required only for infections caused by Legionella pneumophila, Staphylococcus aureus, or Gram-negative enteric bacilli 4.
Switch from IV 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 medications—typically by hospital day 2–3 4.
Critical Timing Considerations
Administer the first antibiotic dose immediately upon diagnosis, ideally in the emergency department 4, 1, 2. Delays beyond 8 hours increase 30-day mortality by 20–30% in hospitalized patients 4, 1.
Common Pitfalls to Avoid
Never use macrolide monotherapy in hospitalized patients, as it fails to cover typical pathogens like S. pneumoniae and leads to treatment failure 4.
Avoid macrolide monotherapy in outpatients when local pneumococcal macrolide resistance exceeds 25% (the situation in most U.S. areas), as this increases risk of breakthrough bacteremia and treatment failure 4.
Do not use indiscriminate fluoroquinolones in uncomplicated outpatient CAP due to FDA warnings about serious adverse events and resistance concerns 4.
Do not add broad-spectrum antipseudomonal or MRSA agents routinely; restrict their use to patients with documented risk factors to avoid unnecessary resistance and adverse effects 4.
Do not delay antibiotic administration while awaiting diagnostic test results, as mortality increases significantly with treatment postponement 1, 3.
Do not rely on "typical vs. atypical" classification of pneumonia based on clinical presentation, as it cannot reliably differentiate pathogen types and coinfection is common 1.
Do not forgo chest radiography in favor of a purely clinical diagnosis; imaging is mandatory to avoid unnecessary antibiotic use for viral bronchitis 1.