What is the definition, signs and symptoms, diagnosis, evaluation, treatment guidelines, and mortality and morbidity factors of Community-Acquired Pneumonia (CAP)?

Community-Acquired Pneumonia: Definition, Diagnosis, Treatment, and Prognostic Factors

Community-acquired pneumonia (CAP) is defined clinically as the presence of signs and symptoms of pneumonia in a previously healthy person due to an infection acquired outside the hospital, which can be verified by radiological findings of consolidation in developed countries. 1

Definition and Clinical Presentation

• CAP is an acute infection of the pulmonary parenchyma acquired in the community, characterized by new radiological shadowing with no other explanation for the illness 1
• Key signs and symptoms include:
  • Fever (>38°C or ≤36°C)
  • New or increased cough
  • Dyspnea
  • Abnormal leukocyte count (<4000/μL or >10,000/μL)
  • Consistent radiographic findings (air space density) 2

Diagnostic Approach

• Diagnosis requires at least two clinical signs or symptoms of pneumonia plus radiographic evidence of infiltrates without an alternative explanation 2
• Chest radiography is essential when symptoms and physical examination suggest pneumonia:
  • Helps differentiate pneumonia from other conditions
  • Can suggest specific etiologies
  • Identifies coexisting conditions like pleural effusion 1
• Laboratory testing should include:
  • Complete blood count
  • Blood urea nitrogen
  • Blood cultures (for hospitalized patients)
  • Sputum Gram stain and culture (controversial but may be useful in some cases) 1
• Testing for COVID-19 and influenza is recommended when these viruses are circulating in the community 2

Etiology

• Common bacterial pathogens:
  • Streptococcus pneumoniae (9-20% of episodes) 1, 3
  • Haemophilus influenzae (3-10% of episodes, especially in smokers) 1, 3
  • Moraxella catarrhalis 4
• Atypical pathogens:
  • Mycoplasma pneumoniae (13-37% of episodes) 1, 3
  • Chlamydia pneumoniae (up to 17% of outpatients) 1, 3
  • Legionella species (0.7-13% of outpatients) 1, 3
• Respiratory viruses (detected in up to 36% of cases) 1, 3
• Mixed infections (bacteria + virus or bacteria + atypical pathogen) occur in approximately 3-10% of cases 3
• No pathogen is identified in 40-70% of CAP cases despite extensive diagnostic testing 1, 3

Severity Assessment

• The CURB-65 score is recommended for assessing CAP severity, including five variables:
  • Confusion
  • Urea nitrogen
  • Respiratory rate
  • Blood pressure
  • Age ≥65 years 5
• CURB-65 scoring interpretation:
  • 0-1 points: consider outpatient treatment
  • 2 points: consider hospital admission
  • ≥3 points: consider ICU admission 5
• The CRB-65 score (omitting blood urea nitrogen) is useful when blood tests are not readily available 5
• The Pneumonia Severity Index (PSI) incorporates 20 variables and is primarily designed to identify low-risk patients who can be safely treated as outpatients 5
• The 2007 IDSA/ATS severe CAP criteria are recommended for identifying patients requiring ICU-level care 5

Treatment Guidelines

Outpatient Treatment

• For patients without cardiopulmonary disease:
  • Oral macrolide (azithromycin) alone
  • If macrolide allergic: Doxycycline or an antipneumococcal fluoroquinolone 1
• For patients with cardiopulmonary disease or modifying factors:
  • Oral β-lactam plus a macrolide or doxycycline
  • Or an antipneumococcal fluoroquinolone alone 1

Hospitalized Non-ICU Patients

• Intravenous β-lactam (cefotaxime, ceftriaxone, ampicillin/sulbactam, high-dose ampicillin) plus intravenous or oral macrolide or doxycycline
• Or intravenous antipneumococcal fluoroquinolone alone 1

ICU Patients

• Without risk for Pseudomonas aeruginosa:
  • Intravenous β-lactam (cefotaxime, ceftriaxone) plus either intravenous macrolide (azithromycin) or intravenous fluoroquinolone 1
• With risk for Pseudomonas aeruginosa:
  • Intravenous antipseudomonal β-lactam (cefepime, imipenem, meropenem, piperacillin/tazobactam) plus intravenous antipseudomonal quinolone (ciprofloxacin) 1

Duration of Treatment

• Minimum of 3 days for hospitalized patients without risk factors for resistant bacteria 2
• Patients should be afebrile for 48-72 hours and clinically stable before discontinuing antibiotics 1

Mortality and Morbidity Factors

• Mortality rates for severe CAP range from 20% to 50% in ICU patients 1
• Nearly all fatal cases develop severe sepsis or septic shock 1
• Risk factors for increased mortality include:
  • Advanced age (≥65 years)
  • Underlying lung disease
  • Smoking
  • Immunosuppression 2
• Complications include:
  • Sepsis
  • Acute respiratory distress syndrome
  • Respiratory failure requiring mechanical ventilation
  • Pleural effusion or empyema 1, 2
• Systemic corticosteroid administration within 24 hours of development of severe CAP may reduce 28-day mortality 2

Prevention

• Pneumococcal vaccination is recommended for adults ≥65 years and those with chronic medical conditions 1
• Annual influenza vaccination is recommended for all persons ≥6 months of age 1
• Smoking cessation reduces the risk of CAP 1