How do I assess severity and choose appropriate antibiotic therapy for a patient with suspected community‑acquired pneumonia?

Assessing Severity and Choosing Antibiotic Therapy for Community-Acquired Pneumonia

Use the CURB-65 or PSI score to stratify severity, then select antibiotics based on treatment setting: outpatients without comorbidities receive amoxicillin 1 g three times daily; outpatients with comorbidities receive combination β-lactam/macrolide or fluoroquinolone monotherapy; hospitalized non-ICU patients receive ceftriaxone 1–2 g IV daily plus azithromycin 500 mg daily; and ICU patients receive ceftriaxone 2 g IV daily plus azithromycin 500 mg IV daily or a respiratory fluoroquinolone. 1, 2

Severity Assessment

Apply Validated Scoring Systems

• Calculate the CURB-65 score by assigning one point each for: Confusion, Urea >7 mmol/L, Respiratory rate ≥30/min, Blood pressure (systolic <90 mmHg or diastolic ≤60 mmHg), and age ≥65 years. 1, 3

• Interpret CURB-65 results: Score 0–1 indicates low risk (outpatient management); score 2 suggests moderate risk (consider hospitalization); score ≥3 indicates high risk (hospitalize, consider ICU). 1, 3

• Alternatively, use the PSI (Pneumonia Severity Index): Classes I–III are appropriate for outpatient care; class IV warrants hospitalization; class V requires inpatient admission. 1, 2

• Recognize that confusion carries the strongest association with 30-day mortality (odds ratio 22.148) among CURB-65 criteria in low-mortality settings, making it a critical red flag. 4

Identify ICU Admission Criteria

• Admit to ICU if any one major criterion is present: septic shock requiring vasopressors OR respiratory failure requiring mechanical ventilation. 1, 2

• Admit to ICU if three or more minor criteria are met: confusion, respiratory rate ≥30/min, systolic blood pressure <90 mmHg, multilobar infiltrates, PaO₂/FiO₂ <250, uremia (BUN >20 mg/dL), leukopenia (WBC <4000/μL), thrombocytopenia (platelets <100,000/μL), hypothermia (temperature <36°C), or need for aggressive fluid resuscitation. 1, 2

• Note that patients transferred to ICU after initial ward admission experience higher mortality than those directly admitted from the emergency department, emphasizing the importance of accurate initial triage. 1

Antibiotic Selection by Clinical Setting

Outpatient Treatment: Previously Healthy Adults

• Prescribe amoxicillin 1 g orally three times daily for 5–7 days as first-line therapy because it retains activity against 90–95% of Streptococcus pneumoniae isolates, including many penicillin-resistant strains. 1, 2

• Use doxycycline 100 mg orally twice daily as an acceptable alternative when amoxicillin is contraindicated, though this carries lower-quality evidence. 1, 2

• Reserve macrolide monotherapy (azithromycin 500 mg day 1, then 250 mg daily; or clarithromycin 500 mg twice daily) only for areas where pneumococcal macrolide resistance is documented <25%; in most U.S. regions resistance is 20–30%, making macrolides unsafe as first-line agents. 1, 2

Outpatient Treatment: Adults with Comorbidities

• Prescribe combination therapy with amoxicillin-clavulanate 875/125 mg orally twice daily PLUS azithromycin (500 mg day 1, then 250 mg daily) OR doxycycline 100 mg twice daily for patients with chronic heart, lung, liver, or renal disease; diabetes; alcoholism; malignancy; or asplenia. 1, 2

• Alternatively, use respiratory fluoroquinolone monotherapy (levofloxacin 750 mg daily OR moxifloxacin 400 mg daily) when β-lactams or macrolides are contraindicated, though fluoroquinolones should be reserved due to FDA safety warnings. 1, 2

Hospitalized Non-ICU Patients

• Administer ceftriaxone 1–2 g IV once daily PLUS azithromycin 500 mg IV or orally daily as the standard regimen, providing coverage for typical bacteria (S. pneumoniae, H. influenzae, M. catarrhalis) and atypical pathogens (Mycoplasma, Chlamydophila, Legionella). 1, 2, 5

• Alternative β-lactams include cefotaxime 1–2 g IV every 8 hours OR ampicillin-sulbactam 3 g IV every 6 hours, always combined with a macrolide. 1, 2

• Respiratory fluoroquinolone monotherapy (levofloxacin 750 mg IV daily OR moxifloxacin 400 mg IV daily) is equally effective and reserved for penicillin-allergic patients. 1, 2

• Obtain blood cultures and sputum Gram stain/culture before initiating antibiotics in all hospitalized patients to enable pathogen-directed therapy. 1, 2

ICU Patients (Severe CAP)

• Mandate combination therapy with ceftriaxone 2 g IV 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 a respiratory fluoroquinolone (levofloxacin 750 mg IV daily or moxifloxacin 400 mg IV daily). 1, 2, 5

• Never use β-lactam monotherapy in ICU patients because it is associated with significantly higher mortality in critically ill patients with bacteremic pneumococcal pneumonia. 1, 2

• For penicillin-allergic ICU patients, use aztreonam 2 g IV every 8 hours PLUS a respiratory fluoroquinolone. 1, 2

Special Pathogen Coverage (Risk-Based)

Pseudomonas aeruginosa Coverage

• Add antipseudomonal therapy ONLY when specific risk factors are present: structural lung disease (bronchiectasis, cystic fibrosis), recent hospitalization with IV antibiotics within 90 days, prior respiratory isolation of P. aeruginosa, or chronic broad-spectrum antibiotic exposure ≥7 days in the past month. 1, 2

• Use 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) for dual antipseudomonal coverage. 1, 2

MRSA Coverage

• Add vancomycin 15 mg/kg IV every 8–12 hours (target trough 15–20 µg/mL) OR linezolid 600 mg IV every 12 hours ONLY when risk factors are present: prior MRSA infection/colonization, recent hospitalization with IV antibiotics, post-influenza pneumonia, or cavitary infiltrates on imaging. 1, 2

Treatment Duration and Transition

Duration of Therapy

• 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, 2, 5

• Typical duration for uncomplicated CAP is 5–7 days. 1, 2, 5

• Extend therapy to 14–21 days ONLY for infections caused by Legionella pneumophila, Staphylococcus aureus, or Gram-negative enteric bacilli. 1, 2

Transition to Oral Therapy

• Switch from IV to oral antibiotics when the patient meets all stability criteria: temperature ≤37.8°C, heart rate ≤100 bpm, respiratory rate ≤24 breaths/min, systolic blood pressure ≥90 mmHg, oxygen saturation ≥90% on room air, able to maintain oral intake, and normal mental status—typically by hospital day 2–3. 1, 2

• Oral step-down options include amoxicillin 1 g three times daily PLUS azithromycin 500 mg daily (or azithromycin alone after 2–3 days of IV therapy). 2

Critical Timing and Pitfalls

Antibiotic Timing

• Administer the first antibiotic dose in the emergency department immediately upon diagnosis; delays beyond 8 hours increase 30-day mortality by 20–30% in hospitalized patients. 1, 2, 5

Common Pitfalls to Avoid

• Never use macrolide monotherapy in hospitalized patients because it fails to cover typical pathogens like S. pneumoniae and leads to treatment failure. 1, 2

• Avoid indiscriminate use of antipseudomonal agents (piperacillin-tazobactam, cefepime) or MRSA coverage (vancomycin, linezolid) unless specific risk factors are documented; routine use promotes resistance without clinical benefit. 1, 2

• Do not use fluoroquinolone monotherapy in ICU patients; combination therapy with a β-lactam is mandatory and reduces mortality. 1, 2

• Avoid oral cephalosporins (cefuroxime, cefpodoxime) as first-line agents because they have inferior in-vitro activity compared with high-dose amoxicillin or IV ceftriaxone. 2

Monitoring and Follow-Up

• Monitor temperature, respiratory rate, pulse, blood pressure, mental status, and oxygen saturation at least twice daily in hospitalized patients to detect early deterioration. 1, 2

• If no clinical improvement by day 2–3, obtain repeat chest radiograph, inflammatory markers (CRP, white blood cell count), and additional microbiologic specimens to evaluate for complications such as pleural effusion, empyema, or resistant organisms. 1, 2

• Schedule routine follow-up at 6 weeks for all patients; obtain chest radiograph only if symptoms persist, physical signs remain, or the patient has high risk for underlying malignancy (smokers >50 years). 1, 2