American Thoracic Society Guidelines for Management of Community‑Acquired Pneumonia in Adults

Diagnostic Criteria

• CAP diagnosis requires the presence of compatible clinical signs/symptoms and radiographic confirmation (e.g., chest X‑ray or CT) to establish the diagnosis. 1, 2, 3

Microbiological Testing

• In hospitalized adults, especially those with severe disease, obtain blood cultures and sputum cultures to identify bacterial pathogens and guide therapy. 1, 2

Severity Assessment & Site‑of‑Care Decision‑Making

• Use validated severity‑of‑illness scores (PSI/PORT or CURB‑65) to determine the appropriate level of care:
  
  • Low‑risk patients → outpatient management.
  • Moderate‑risk patients with comorbidities or abnormal vitals → admission to a general ward.
  • Severe CAP (need for vasopressors, mechanical ventilation, or ≥3 minor criteria) → ICU admission. 1, 2

Empiric Antibiotic Regimens

Outpatient (Previously Healthy, No DRP Risk)

• First‑line therapy is amoxicillin monotherapy. 1

Outpatient (Comorbidities or Risk Factors for Drug‑Resistant Pathogens)

• Combination of amoxicillin‑clavulanate or a cephalosporin plus a macrolide, or monotherapy with a respiratory fluoroquinolone (levofloxacin or moxifloxacin). 1

Hospitalized Non‑ICU Patients

• Standard regimen: a β‑lactam (ceftriaxone, cefotaxime, or ampicillin‑sulbactam) plus a macrolide.
• Alternative: monotherapy with a respiratory fluoroquinolone. 1, 2

Severe CAP (ICU Patients)

• Mandatory combination therapy: a β‑lactam (ceftriaxone or cefotaxime) plus either azithromycin or a respiratory fluoroquinolone. 1

Coverage for Specific High‑Risk Pathogens

• Pseudomonas aeruginosa risk (e.g., structural lung disease, recent hospitalization/antibiotics): anti‑pseudomonal β‑lactam (piperacillin‑tazobactam, cefepime, imipenem, or meropenem) plus ciprofloxacin or high‑dose levofloxacin; alternatively, add an aminoglycoside and azithromycin/respiratory fluoroquinolone. 1
• MRSA risk (e.g., prior MRSA infection, IV drug use, necrotizing pneumonia): add vancomycin or linezolid to the above regimen. 1

Antibiotic Duration

• Minimum effective course is 5 days for patients who are clinically stable and afebrile for 48–72 hours; shorter courses (5–7 days) are preferred over traditional 10–14 day regimens.
• Extend duration for complications such as empyema, slow‑resolving bacteremia, or infections caused by Staphylococcus aureus or Pseudomonas aeruginosa. 1

Adjunctive Corticosteroid Therapy

• Routine systemic corticosteroids are not recommended for all CAP patients.
• Consider corticosteroids only in severe CAP with refractory septic shock, following standard sepsis protocols; evidence is mixed, with potential benefits offset by risks of hyperglycemia and secondary infection. 1

Follow‑up Imaging

• Routine repeat chest imaging is unnecessary for patients who achieve clinical stability.
• Obtain follow‑up imaging (6–12 weeks) only for those with persistent symptoms, high‑risk features (age > 50 years, smoking history), or initial findings suggestive of underlying malignancy. 1, 2

Antimicrobial Stewardship Pitfalls

• Do not withhold antibacterial therapy solely because a respiratory viral test is positive; bacterial co‑infection remains common.
• Avoid fluoroquinolone monotherapy in outpatients without comorbidities to limit resistance development. 1

Common Pathogens Targeted by Empiric Therapy

• Streptococcus pneumoniae (most frequent)
• Haemophilus influenzae
• Mycoplasma pneumoniae
• Staphylococcus aureus (including MRSA in high‑risk groups)
• Legionella spp.
• Chlamydia pneumoniae
• Moraxella catarrhalis
• Pseudomonas aeruginosa (in patients with specific risk factors) 1, 2

ATS/IDSA Guideline Recommendations for Antibiotic Selection and Diagnostic Testing in Community‑Acquired Pneumonia

Outpatient Management (Previously Healthy Adults)

• Amoxicillin 1 g three times daily is the preferred first‑line agent for uncomplicated CAP because it provides superior coverage of Streptococcus pneumoniae compared with macrolides or doxycycline. (ATS/IDSA) 4
• Doxycycline 100 mg twice daily is an acceptable alternative when β‑lactam allergy or intolerance precludes amoxicillin use. (ATS/IDSA) 4
• Macrolide monotherapy (azithromycin or clarithromycin) may be used only if local pneumococcal resistance is < 25 % to preserve efficacy and limit resistance. (ATS/IDSA) 4

Outpatient Management with Comorbidities or Risk Factors

• Combination therapy – either amoxicillin/clavulanate (875 mg/125 mg BID or 2 g/125 mg BID) or a respiratory cephalosporin (cefpodoxime 200 mg BID or cefuroxime 500 mg BID) plus a macrolide or doxycycline is recommended for patients with chronic heart, lung, liver, renal disease, diabetes, alcoholism, malignancy, or asplenia. (ATS/IDSA) 4
• Respiratory fluoroquinolone monotherapy (levofloxacin 750 mg daily, moxifloxacin 400 mg daily, or gemifloxacin 320 mg daily) is an alternative regimen for the same high‑risk outpatient group. (ATS/IDSA) 4

Diagnostic Testing in Hospitalized or Severe CAP

• Urine pneumococcal antigen testing is not routinely performed; it is reserved for patients with severe CAP where rapid identification may influence management. (ATS/IDSA) 4
• Legionella urinary antigen testing should be ordered only for severe CAP or when epidemiologic clues (outbreak, recent travel) suggest Legionella infection. (ATS/IDSA) 4
• In severe CAP, lower‑respiratory‑tract specimens (e.g., sputum, bronchoalveolar lavage) should be obtained for Legionella culture or nucleic‑acid amplification testing to improve pathogen detection. (ATS/IDSA) 4

Coverage of High‑Risk Pathogens in ICU Patients

• Pseudomonas aeruginosa risk (structural lung disease, recent hospitalization with parenteral antibiotics, bronchiectasis) warrants an anti‑pseudomonal β‑lactam (piperacillin‑tazobactam, cefepime, imipenem, or meropenem) combined with a fluoroquinolone (ciprofloxacin or high‑dose levofloxacin). (ATS/IDSA) 4
• MRSA risk (prior MRSA respiratory isolate, IV drug use, necrotizing pneumonia, recent hospitalization with parenteral antibiotics) requires addition of vancomycin or linezolid to the β‑lactam‑plus‑macrolide/fluoroquinolone backbone. (ATS/IDSA) 4

Targeted Pathogens Emphasized by the Guidelines

All statements are derived from the ATS/IDSA 2019 community‑acquired pneumonia guideline. 4