What is the definition of hospital-acquired pneumonia (HAP)?

Definition of Hospital-Acquired Pneumonia (HAP)

Hospital-acquired pneumonia (HAP) is defined as pneumonia that occurs 48 hours or more after hospital admission, which was not incubating at the time of admission. 1

Key Diagnostic Criteria

HAP is characterized by:

• New or progressive lung infiltrates on chest radiography 1
• Clinical evidence that the infiltrate is of infectious origin, including:
  • Fever (temperature > 38.3°C) without other cause 1
  • Leukocytosis (>12,000/mm³) or leukopenia (<4,000/mm³) 1
  • And at least two of the following:
    • Purulent sputum 1
    • Cough or dyspnea 1
    • Declining oxygenation or increased oxygen requirement 1

Subtypes of HAP

HAP can be further categorized into specific subtypes:

• Non-ventilator HAP: Occurs after 48 hours of hospital stay in non-ventilated patients 1
• Ventilator-associated pneumonia (VAP): Develops in ICU patients who have been mechanically ventilated for at least 48 hours 1
  • Note: Patients who require intubation after developing severe HAP do not meet the definition of VAP but should be managed similarly 1

Timing Classification

HAP can be classified based on onset relative to hospital admission:

• Early-onset HAP: Develops within the first 5 days of hospitalization 1
• Late-onset HAP: Develops on or after day 5 of hospitalization 1

This timing distinction is important as it affects the likely pathogens involved and guides empiric antibiotic therapy decisions.

Epidemiology and Impact

HAP is associated with significant morbidity and mortality:

• Second most common nosocomial infection after bloodstream infections 1, 2
• Most common infection acquired in intensive care units 1
• Leading cause of death from nosocomial infections in critically ill patients 1, 3
• Incidence ranges from 5 to more than 20 cases per 1000 hospital admissions 1
• Associated with increased duration of mechanical ventilation, ICU stay, and hospital length of stay 1
• Significantly increases healthcare costs, with estimates ranging from $10,000 to $40,000 per case 1

Common Pathogens

The microbiology of HAP varies based on timing of onset:

• Early-onset HAP: Usually caused by community-acquired pathogens 1, 3

  • Methicillin-susceptible Staphylococcus aureus
  • Streptococcus pneumoniae
  • Haemophilus influenzae

• Late-onset HAP: More likely to involve multidrug-resistant organisms 1, 3

  • Enterobacteriaceae
  • Pseudomonas aeruginosa
  • Acinetobacter baumannii
  • Methicillin-resistant Staphylococcus aureus

Polymicrobial infection occurs in approximately 30% of cases 1.

Diagnostic Approach

Microbiological confirmation is crucial for definitive diagnosis:

• Based on qualitative or quantitative cultures of respiratory samples 1
• Pathogens are identified in approximately 70% of suspected cases 1
• Supplementary tests include blood cultures and antigen detection in urine 1

Understanding this definition and classification of HAP is essential for early recognition, appropriate diagnostic workup, and effective management to reduce associated morbidity and mortality.