What is the definition of hospital‑acquired pneumonia (HAP) and how does it differ from ventilator‑associated pneumonia (VAP)?

Definition of Hospital-Acquired Pneumonia

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

Core Diagnostic Criteria

HAP requires the presence of:

• New or progressive lung infiltrates on chest radiography 1
• Clinical evidence of infection, including fever, leukocytosis or leukopenia 1
• At least two respiratory symptoms: purulent sputum, cough or dyspnea, declining oxygenation, or increased oxygen requirement 1

The 48-hour threshold is critical—any pneumonia developing before this timeframe is considered community-acquired or incubating at admission. 1, 2

Key Distinction: HAP vs VAP

HAP encompasses two distinct clinical entities that differ fundamentally in their epidemiology and management:

Non-Ventilator HAP

• Occurs in non-ventilated patients after 48 hours of hospital stay 3, 1
• Can develop in general hospital wards or ICU settings 3
• Incidence approximately 1.6% of hospitalized patients (3.63 per 1000 patient-days) 3

Ventilator-Associated Pneumonia (VAP)

• Develops in mechanically ventilated patients more than 48 hours after intubation 3, 1
• The timing is measured from initiation of mechanical ventilation, not hospital admission 3
• Incidence ranges from 1.9 to 3.8 per 1000 ventilator-days in the US, exceeding 18 per 1000 ventilator-days in Europe 3
• Cumulative risk approximately 1% per day of mechanical ventilation 4

The critical distinction is that VAP timing is defined by duration of mechanical ventilation (≥48 hours), while non-ventilator HAP is defined by duration of hospitalization (≥48 hours). 3, 1

Clinical Impact and Mortality

HAP carries significant morbidity and mortality burden:

• Overall mortality rate approximately 20% 3, 1
• Attributable mortality 5-13% (mortality directly caused by the pneumonia itself) 3, 1
• Attributable mortality may reach 30-33% in some populations 2
• VAP and ventilated HAP mortality ranges 15-30% depending on severity 5

The attributable mortality is highest with multidrug-resistant organisms, particularly Pseudomonas aeruginosa and methicillin-resistant Staphylococcus aureus (MRSA). 6

Microbiological Patterns

The causative pathogens differ based on timing:

Early-Onset HAP (within 5 days of hospitalization)

• Typically caused by community-acquired pathogens 1
• Common organisms: methicillin-susceptible Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae 1

Late-Onset HAP (after 5 days of hospitalization)

• More likely to involve multidrug-resistant organisms 1
• Common organisms: Enterobacteriaceae, Pseudomonas aeruginosa, Acinetobacter baumannii, MRSA 1

Microbiological confirmation is crucial for definitive diagnosis, with pathogens identified in approximately 70% of suspected cases using qualitative or quantitative cultures of respiratory samples. 1

Important Clinical Caveats

The "Not Incubating" Criterion

• HAP must not have been incubating at admission 1, 2
• This distinguishes it from community-acquired pneumonia that manifests shortly after admission 2
• Infections appearing within 48 hours after discharge may still be considered nosocomial if linked to the hospital stay 2

Risk Factors Affecting Pathogen Spectrum

Even in early-onset HAP, prior antibiotic exposure and previous hospitalization increase risk for multidrug-resistant organisms, regardless of timing. 6 This means the traditional early/late distinction may be insufficient—individual risk assessment is essential. 1

Diagnostic Pitfalls

• Multidrug-resistant pathogens like MRSA may be mistakenly considered community-acquired if previous hospital stays were not documented 2
• Resistance rates are substantially higher in nosocomial versus community-acquired infections, making this distinction crucial for antibiotic selection 2