What are the classifications of community‑acquired pneumonia?

Community-Acquired Pneumonia Classification

Community-acquired pneumonia is classified primarily by site of care and severity into four main categories: outpatient (low-risk), inpatient non-ICU (moderate severity), inpatient ICU (severe), and ICU with risk factors for resistant pathogens (severe with complications). 1

Classification by Site of Care and Severity

Outpatient (Low-Risk) CAP

• Risk Class I-III patients who can be safely managed at home 1
• Previously healthy individuals without comorbidities 1
• Patients with comorbidities (COPD, diabetes, renal/heart failure, malignancy) but stable presentation 1
• Most common pathogens: Streptococcus pneumoniae, Mycoplasma pneumoniae, Haemophilus influenzae, Chlamydophila pneumoniae, respiratory viruses 1

Inpatient Non-ICU (Moderate Severity) CAP

• Risk Class IV-V patients requiring hospitalization but not intensive care 1, 2
• Patients with significant comorbidities or abnormal vital signs 1
• Most common pathogens: S. pneumoniae, M. pneumoniae, C. pneumoniae, H. influenzae, Legionella species, aspiration, respiratory viruses 1

Inpatient ICU (Severe) CAP

• Patients meeting severe CAP criteria requiring intensive care unit admission 1
• Defined by presence of either:
  • Major criteria: Requiring invasive mechanical ventilation OR septic shock requiring vasopressors 1
  • Minor criteria (≥3 of the following): Respiratory rate ≥30/min, PaO₂/FiO₂ ratio ≤250, multilobar infiltrates, confusion/disorientation, uremia (BUN ≥20 mg/dL), leukopenia (WBC <4,000 cells/mm³), thrombocytopenia (platelets <100,000/mm³), hypothermia (core temperature <36°C), hypotension requiring aggressive fluid resuscitation 1
• Most common pathogens: S. pneumoniae, Staphylococcus aureus, Legionella species, gram-negative bacilli, H. influenzae 1
• Mortality rate: 20-50% depending on admission criteria and presence of septic shock 1

ICU with Risk Factors for Resistant Pathogens

• Severe CAP with risk factors for Pseudomonas aeruginosa: Structural lung disease (bronchiectasis), recent hospitalization with parenteral antibiotics within 90 days, chronic/prolonged broad-spectrum antibiotic therapy (≥7 days in past month) 1
• Severe CAP with risk factors for MRSA: Prior MRSA infection/colonization, recent hospitalization with parenteral antibiotics within 90 days, prior respiratory isolation of MRSA 1
• Pathogens include: All severe CAP pathogens plus P. aeruginosa and/or MRSA 1

Classification by Pneumonia Severity Index (PSI)

The PSI stratifies patients into five risk classes based on mortality risk 1:

Risk Class I

• Age <50 years
• No comorbidities (neoplastic disease, liver disease, CHF, cerebrovascular disease, renal disease)
• Normal or mildly deranged vital signs
• Normal mental status 1

Risk Classes II-V

• Calculated using point system based on:

  • Demographics: Age (years = points for men; age minus 10 for women), nursing home residency (+10 points) 1
  • Comorbidities: Neoplastic disease (+30), liver disease (+20), CHF (+10), cerebrovascular disease (+10), renal disease (+10) 1
  • Physical examination findings: Altered mental status (+20), respiratory rate ≥30/min (+20), systolic BP <90 mmHg (+20), temperature <35°C or ≥40°C (+15), pulse ≥125/min (+10) 1
  • Laboratory/radiographic findings: pH <7.35 (+30), BUN ≥30 mg/dL (+20), sodium <130 mEq/L (+20), glucose ≥250 mg/dL (+10), hematocrit <30% (+10), PaO₂ <60 mmHg (+10), pleural effusion (+10) 1

• Risk Class II: ≤70 points

• Risk Class III: 71-90 points

• Risk Class IV: 91-130 points

• Risk Class V: >130 points 1, 2

Alternative Severity Classification: CURB-65

CURB-65 score assigns one point for each criterion 1:

• Confusion (new onset)
• Urea >7 mmol/L (BUN >19 mg/dL)
• Respiratory rate ≥30/min
• Blood pressure (systolic <90 mmHg or diastolic ≤60 mmHg)
• Age ≥65 years

Score interpretation: 0-1 points = outpatient treatment; 2 points = consider hospitalization; ≥3 points = hospitalization, consider ICU 1

Common Pitfalls in Classification

• PSI Class V patients are heterogeneous: 80% are managed on general wards despite high-risk classification; ICU admission depends more on acute illness severity than comorbidity burden 2
• Age bias in PSI: Older patients automatically score higher due to age points, potentially leading to over-hospitalization of stable elderly patients 2
• Clinical judgment remains essential: Severity scores should guide but not replace physician assessment, particularly for younger patients with severe acute illness 1
• Mixed infections are common: Up to 40% of CAP cases involve coinfection with bacterial and atypical pathogens, though clinical significance remains debated 1