What is the initial treatment for an adult diagnosed with community-acquired pneumonia?

Initial Treatment for Community-Acquired Pneumonia in Adults

For outpatient adults without comorbidities, amoxicillin 1 g three times daily is the preferred first-line therapy, while hospitalized non-ICU patients should receive ceftriaxone 1-2 g IV daily plus azithromycin 500 mg daily, and ICU patients require mandatory combination therapy with a β-lactam plus either azithromycin or a respiratory fluoroquinolone. 1

Outpatient Treatment Algorithm

Healthy Adults Without Comorbidities

• Amoxicillin 1 g orally three times daily is the preferred first-line agent based on effectiveness against common CAP pathogens and moderate quality evidence 1
• Doxycycline 100 mg orally twice daily serves as an acceptable alternative (conditional recommendation) 1, 2
• Macrolides (azithromycin 500 mg on day 1, then 250 mg daily; or clarithromycin 500 mg twice daily) should only be used in areas where pneumococcal macrolide resistance is <25% 1, 3

Adults With Comorbidities

• Combination therapy is required: β-lactam (amoxicillin-clavulanate, cefpodoxime, or cefuroxime) plus macrolide (azithromycin or clarithromycin) or doxycycline 1
• Alternative: Respiratory fluoroquinolone monotherapy (levofloxacin 750 mg daily, moxifloxacin 400 mg daily, or gemifloxacin) 1, 4
• Comorbidities include chronic heart, lung, liver, or renal disease, diabetes, alcoholism, malignancy, or asplenia 5

Inpatient Non-ICU Treatment

Standard Regimen

• β-lactam plus macrolide combination is strongly recommended: ceftriaxone 1-2 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 daily (strong recommendation, high quality evidence) 1, 3
• Alternative: Respiratory fluoroquinolone monotherapy (levofloxacin 750 mg IV daily or moxifloxacin 400 mg IV daily) with equivalent efficacy (strong recommendation, high quality evidence) 1, 4

Penicillin-Allergic Patients

• Use respiratory fluoroquinolone monotherapy 1
• Alternative: Aztreonam 2 g IV every 8 hours plus azithromycin 500 mg IV daily 1

Transition to Oral Therapy

• Switch from IV to oral when patients are hemodynamically stable, clinically improving, able to take oral medications, and have normal GI function, typically by day 2-3 1
• Oral step-down: Amoxicillin 1 g orally three times daily plus azithromycin 500 mg orally daily 1

ICU Treatment for Severe CAP

Mandatory Combination Therapy

• β-lactam (ceftriaxone 2 g IV daily, cefotaxime 1-2 g IV every 8 hours, or ampicillin-sulbactam 3 g IV every 6 hours) PLUS either azithromycin 500 mg daily OR respiratory fluoroquinolone (levofloxacin 750 mg IV daily or moxifloxacin 400 mg IV daily) (strong recommendation) 1, 6
• This dual coverage is obligatory for all ICU patients regardless of other factors 1

Risk Factors for Pseudomonas aeruginosa

• Structural lung disease (bronchiectasis), recent hospitalization with parenteral antibiotics within 90 days, prior respiratory isolation of P. aeruginosa, or recent broad-spectrum antibiotic use (≥7 days within past month) 6, 1
• Antipseudomonal regimen: Antipseudomonal β-lactam (piperacillin-tazobactam, cefepime, imipenem, or meropenem) PLUS ciprofloxacin 400 mg IV every 8 hours OR levofloxacin 750 mg IV daily 6, 1
• Alternative: Antipseudomonal β-lactam PLUS aminoglycoside (gentamicin 5-7 mg/kg IV daily or tobramycin 5-7 mg/kg IV daily) PLUS azithromycin or antipneumococcal fluoroquinolone 6, 1

Risk Factors for MRSA

• Prior MRSA infection or colonization, recent hospitalization with parenteral antibiotics, cavitary infiltrates on imaging, or concurrent influenza 1
• Add vancomycin 15 mg/kg IV every 8-12 hours (target trough 15-20 mg/mL) OR linezolid 600 mg IV every 12 hours to the base regimen 1

Duration of Therapy

• Minimum of 5 days for uncomplicated CAP once clinical stability is achieved (afebrile for 48-72 hours with no more than one sign of clinical instability) 1, 3
• Standard duration: 5-7 days for most cases 1
• Extended duration of 14-21 days for Legionella, Staphylococcus aureus, or Gram-negative enteric bacilli 1

Critical Diagnostic Considerations

• Obtain blood and sputum cultures before initiating antibiotics in all hospitalized patients 1
• Test all patients for COVID-19 and influenza when these viruses are common in the community, as diagnosis may affect treatment and infection prevention strategies 3
• Chest radiography (PA and lateral) is valuable for confirming diagnosis, identifying complications (lung abscess, pleural effusion), and assessing severity (multilobar involvement) 6

Key Clinical Pitfalls to Avoid

• Avoid macrolide monotherapy in areas with >25% pneumococcal macrolide resistance to prevent treatment failure 1
• Avoid delayed antibiotic administration in hospitalized patients—administer the first dose in the emergency department, as delays increase mortality risk 1
• Do not use cefuroxime, cefepime, piperacillin-tazobactam, or carbapenems as first-line empiric therapy unless specific risk factors for Pseudomonas or MRSA are present 1
• Do not extend therapy beyond 7 days in responding patients without specific indications, as this increases resistance risk 1
• Avoid automatically escalating to broad-spectrum antibiotics based solely on immunosuppression without documented risk factors 1