What are the types and treatment options for community-acquired pneumonia (CAP)?

Types and Treatment of Community-Acquired Pneumonia

Classification by Etiology

Community-acquired pneumonia is classified into bacterial, atypical bacterial, and viral types, with bacterial pathogens including Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus, and Moraxella catarrhalis; atypical pathogens including Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella pneumophila; and viral pathogens increasingly recognized as causes of CAP. 1

Bacterial Pathogens (Traditional)

• Streptococcus pneumoniae remains the most common bacterial cause, identified in approximately 15% of patients with confirmed etiology, though the microbiology is changing with widespread pneumococcal conjugate vaccine use 1, 2
• Haemophilus influenzae and Moraxella catarrhalis are common in patients with underlying lung disease 1
• Staphylococcus aureus (including methicillin-resistant strains) should be considered, particularly in patients with risk factors such as prior MRSA infection, recent hospitalization, or concurrent influenza 1, 3
• Gram-negative enteric bacilli including Pseudomonas aeruginosa occur in patients with structural lung disease or recent broad-spectrum antibiotic exposure 1, 3

Atypical Bacterial Pathogens

• Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella pneumophila collectively account for approximately 22% of CAP cases worldwide 4
• These pathogens share many clinical features with typical bacterial pneumonia, making laboratory identification essential for definitive diagnosis 4
• Atypical pathogens require coverage with macrolides, fluoroquinolones, or doxycycline, as β-lactam monotherapy is ineffective 1, 4

Viral Pathogens

• Viruses are identified in up to 40% of hospitalized CAP patients with confirmed etiology, representing an increasingly frequent cause 1, 2
• Influenza A and COVID-19 should be tested when these viruses are circulating in the community, as their diagnosis affects treatment and infection prevention strategies 2

Classification by Severity and Site of Care

Outpatient CAP (Low Severity)

• Healthy adults without comorbidities: First-line treatment is amoxicillin 1 g three times daily 3, 5
• Alternative options include doxycycline 100 mg twice daily or macrolides (azithromycin 500 mg day 1, then 250 mg daily; or clarithromycin 500 mg twice daily) only in areas with pneumococcal macrolide resistance <25% 3, 5
• Adults with comorbidities (chronic heart, lung, liver, or renal disease; diabetes; alcoholism; malignancy; asplenia): Combination therapy with β-lactam (amoxicillin/clavulanate 875/125 mg twice daily OR cefpodoxime 200 mg twice daily OR cefuroxime 500 mg twice daily) PLUS macrolide or doxycycline 3, 5
• Respiratory fluoroquinolone monotherapy (levofloxacin 750 mg daily or moxifloxacin 400 mg daily) is an alternative for patients with comorbidities 3, 5

Inpatient CAP (Moderate Severity, Non-ICU)

• Preferred regimen: β-lactam (ampicillin/sulbactam, ceftriaxone 1-2 g daily, cefotaxime, or ceftaroline) PLUS macrolide (azithromycin 500 mg daily or clarithromycin) 1, 3, 5
• Alternative regimen: Respiratory fluoroquinolone monotherapy (levofloxacin 750 mg daily or moxifloxacin 400 mg daily) 3, 5
• β-lactam plus doxycycline is a lower-quality evidence alternative for patients with contraindications to both macrolides and fluoroquinolones 5

Severe CAP Requiring ICU Admission

• Without Pseudomonas risk factors: Non-antipseudomonal β-lactam (ceftriaxone or cefotaxime) PLUS either azithromycin OR respiratory fluoroquinolone 3, 5
• With Pseudomonas risk factors (structural lung disease, recent hospitalization with parenteral antibiotics, prior P. aeruginosa isolation, recent broad-spectrum antibiotic use): Antipseudomonal β-lactam (cefepime, ceftazidime, piperacillin-tazobactam, or meropenem) PLUS either ciprofloxacin OR levofloxacin OR (aminoglycoside PLUS azithromycin) 3, 5
• With MRSA risk factors (prior MRSA infection/colonization, recent hospitalization with parenteral antibiotics, cavitary infiltrates, concurrent influenza): Add vancomycin or linezolid to base regimen 3, 5

Treatment Duration and Transition Strategy

Duration of Therapy

• Minimum duration: 5 days for uncomplicated CAP in responding patients 6, 3, 5
• Patients must meet specific criteria before discontinuation: improvement in cough and dyspnea, afebrile status (or only isolated fever with other favorable clinical features), decreasing white blood cell count, and functioning gastrointestinal tract 6, 3
• Longer courses (7-14 days) may be required for severe infections, specific pathogens (such as Legionella), or complications 3, 5

Transition from IV to Oral Therapy

• Switch when patients are hemodynamically stable, clinically improving, able to take oral medications, and have normal gastrointestinal function 6, 5
• Transition typically occurs by day 2-3 of hospitalization if clinical stability criteria are met 5
• Even if the patient remains febrile, transition can occur if other clinical features are favorable 6

Critical Pitfalls to Avoid

Diagnostic and Testing Errors

• Do not withhold initial antibiotic therapy based on procalcitonin levels in patients with clinically suspected and radiographically confirmed CAP 3
• Obtain blood and sputum cultures before initiating antibiotics in all hospitalized patients, particularly those empirically treated for MRSA or P. aeruginosa, to allow targeted de-escalation 1, 5
• Test all patients for COVID-19 and influenza when these viruses are circulating in the community 2

Treatment Selection Errors

• Avoid macrolide monotherapy in areas with pneumococcal macrolide resistance ≥25% due to treatment failure risk 3, 5
• Do not automatically escalate to broad-spectrum antibiotics for immunosuppression alone (such as tyrosine kinase inhibitor use) without documented risk factors for resistant pathogens 5
• Abandon the healthcare-associated pneumonia categorization; only cover empirically for MRSA or P. aeruginosa if locally validated risk factors are present 6, 5

Antibiotic Stewardship Errors

• Administer the first antibiotic dose in the emergency department for hospitalized patients, as delayed administration increases mortality 5
• If cultures for P. aeruginosa or MRSA are negative and the patient is improving, narrow expanded therapy within 48 hours of starting treatment 1
• Do not extend therapy beyond 7 days in responding patients without specific indications, as this increases antimicrobial resistance risk 5

Special Pathogen Considerations

Legionella Species

• Preferred treatment: Respiratory fluoroquinolone (levofloxacin preferred) or macrolide (azithromycin preferred) ± rifampin 3
• Clinical success rates for Legionella pneumophila are approximately 70% with appropriate therapy 7

Multi-Drug Resistant Streptococcus pneumoniae (MDRSP)

• MDRSP isolates are resistant to ≥2 of the following: penicillin (MIC ≥2 mcg/mL), 2nd generation cephalosporins, macrolides, tetracyclines, trimethoprim/sulfamethoxazole 1
• Levofloxacin achieves 95% clinical and bacteriologic success rates against MDRSP 7
• The 7-14 day CAP treatment regimen is indicated for MDRSP, while the 5-day regimen excludes MDRSP isolates 7

COVID-19 Related Pneumonia

• Empirical antibacterial coverage is not required in all patients with confirmed COVID-19 pneumonia, unlike CAP without confirmed COVID-19 1
• When bacterial co-infection is suspected in COVID-19 patients, the same bacterial pathogens and empirical antibiotic recommendations apply as for other CAP patients 1
• Procalcitonin may be helpful in limiting antibiotic overuse in COVID-19-related pneumonia 1