What are the Infectious Diseases Society of America (IDSA) guidelines for diagnosing and treating pneumonia?

IDSA/ATS Guidelines for Community-Acquired Pneumonia

Diagnostic Criteria

Community-acquired pneumonia is diagnosed when a patient presents with acute respiratory symptoms (fever, cough, dyspnea, chest discomfort) plus radiographic evidence of a new pulmonary infiltrate, in someone not hospitalized or residing in long-term care for >14 days before symptom onset. 1

Clinical Presentation Requirements

• At least 2 of the following symptoms: fever or hypothermia, rigors, sweats, new cough with or without sputum, change in respiratory secretions, chest discomfort, or dyspnea 1
• Altered breath sounds or localized rales on examination 1
• Chest radiograph showing new infiltrate (standard for confirmation, though clinical diagnosis alone acceptable in mild outpatient cases) 2

Key Diagnostic Limitation

• Clinical characteristics cannot reliably establish specific etiological diagnosis—the "typical" versus "atypical" classification has limited clinical value 2
• Up to 50% of CAP cases never have a pathogen identified despite extensive testing 2

Severity Assessment

Use validated severity scores (PSI or CURB-65) to determine hospitalization need and treatment intensity. 2

Severe CAP Criteria (Major Criteria)

• Septic shock requiring vasopressors 1
• Respiratory failure requiring mechanical ventilation 1

Diagnostic Testing Recommendations

Blood Cultures

• Do NOT obtain in outpatients (strong recommendation) 1
• Do NOT routinely obtain in hospitalized patients with non-severe CAP (conditional recommendation) 1
• DO obtain pretreatment blood cultures in:
  • Severe CAP (strong recommendation) 1
  • Patients empirically treated for MRSA or Pseudomonas aeruginosa (strong recommendation) 1
  • Prior MRSA or P. aeruginosa infection, especially respiratory (conditional recommendation) 1
  • Hospitalization with parenteral antibiotics in last 90 days (conditional recommendation) 1

Rationale: Blood culture yield is only 2-9% in non-severe CAP, rarely changes empiric therapy, and false-positives increase length of stay and unnecessary antibiotic use 1

Sputum Cultures

• Do NOT obtain in outpatients 2
• DO obtain in:
  • Severe CAP 1
  • All inpatients empirically treated for MRSA or P. aeruginosa 1
  • Prior MRSA or P. aeruginosa infection 1
  • Hospitalization with parenteral antibiotics in last 90 days 1

Urine Antigen Testing

• Do NOT routinely test for pneumococcal antigen except in severe CAP 1, 2
• Do NOT routinely test for Legionella antigen except in: 1, 2
  • Severe CAP
  • Legionella outbreak or recent travel
  • Consider Legionella culture or nucleic acid testing in severe CAP 1

Procalcitonin

• Do NOT use to determine need for initial antibacterial therapy 1

Empiric Antibiotic Treatment

Initiate empiric antibiotics immediately based on severity and risk factors without delaying for diagnostic testing. 2

Outpatient Non-Severe CAP

• First-line: Amoxicillin 500-1000 mg every 8 hours orally 2, 3
• Alternative: Macrolide monotherapy (conditional recommendation due to resistance concerns) 1
• Alternative: Doxycycline 4
• Alternative: Respiratory fluoroquinolone 4

Hospitalized Non-Severe CAP

• Preferred: β-lactam (amoxicillin, ampicillin, ceftriaxone, or cefuroxime) PLUS macrolide (azithromycin, clarithromycin, or erythromycin) 2, 3
• Alternative: Respiratory fluoroquinolone monotherapy 4

Severe CAP (ICU)

• Preferred: β-lactam (ceftriaxone, cefotaxime, or ampicillin-sulbactam) PLUS macrolide 1, 2
  • Stronger evidence supports β-lactam/macrolide over β-lactam/fluoroquinolone 1
• Alternative: β-lactam PLUS respiratory fluoroquinolone 1

MRSA Coverage Indications

Add vancomycin or linezolid when: 2

• Prior MRSA infection (especially respiratory)
• Recent hospitalization with IV antibiotics
• High local MRSA prevalence

Pseudomonas aeruginosa Coverage Indications

Add anti-pseudomonal β-lactam (piperacillin-tazobactam, cefepime, imipenem, or meropenem) when: 2

• Prior Pseudomonas infection
• Structural lung disease (bronchiectasis, severe COPD)
• Recent hospitalization with IV antibiotics in last 90 days

Key Changes from 2007 Guidelines

The 2019 update made several critical revisions: 1

• Abandoned healthcare-associated pneumonia (HCAP) category—now emphasize local epidemiology and validated risk factors
• Macrolide monotherapy downgraded from strong to conditional recommendation due to resistance
• Expanded indications for sputum/blood cultures to include empiric MRSA/Pseudomonas treatment
• Corticosteroids NOT recommended except possibly in refractory septic shock
• Increased emphasis on antibiotic de-escalation if cultures negative

Treatment Duration and Follow-Up

• Minimum 5 days of therapy for all patients with clinical stability required before discontinuation 2
• Severe CAP without identified pathogen: 10 days 3
• Evaluate non-severe CAP at day 5-7 for symptom improvement 3
• Evaluate severe CAP at day 2-3 for fever reduction and lack of infiltrate progression 3

Follow-Up Imaging

• Do NOT obtain routine follow-up chest radiographs in patients achieving clinical stability 1, 2
• Consider lung cancer screening if patient meets eligibility criteria 1

Performance Indicators

The 2000 IDSA guidelines established quality metrics: 1

• Blood cultures obtained before antibiotics
• Antibiotics initiated within 8 hours of hospitalization
• Legionella testing in ICU patients
• Pulse oximetry or blood gases within 24 hours of admission

Critical Pitfalls to Avoid

• Never delay antibiotics in severe CAP waiting for diagnostic results—give immediately after diagnosis 3
• Avoid underdosing amoxicillin—use 500-1000 mg every 8 hours, not lower doses 3
• Do not rely on "typical" versus "atypical" clinical features to guide pathogen-specific therapy 2
• Beware false-positive blood cultures (coagulase-negative staph) leading to unnecessary antibiotic escalation 1
• Recognize atypical presentations in elderly—may lack fever despite serious infection 3