Definition of Hospital-Acquired Pneumonia (HAP)
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 Components
HAP requires the presence of new lung infiltrate plus clinical evidence that the infiltrate is of infectious origin, which includes:
Critical Temporal Distinction
The 48-hour threshold is fundamental to the definition and serves to distinguish HAP from community-acquired pneumonia. 1, 2 This timeframe accounts for the typical incubation period of nosocomial pathogens and ensures that infections present at admission are not misclassified. 3
Important Terminology Clarification
In the 2016 IDSA/ATS guidelines, the term "HAP" specifically denotes pneumonia NOT associated with mechanical ventilation. 1 This creates two mutually exclusive categories:
- HAP: Non-ventilator-associated hospital pneumonia 1
- VAP (Ventilator-Associated Pneumonia): Pneumonia occurring >48 hours after endotracheal intubation 1
This distinction is critical because some literature uses "HAP" as an umbrella term that includes VAP, while the current standard separates them. 1
Temporal Subclassification
- Early-onset HAP: Occurs within the first 5 days of hospitalization, typically caused by community-acquired pathogens (methicillin-susceptible Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae) 2, 4
- Late-onset HAP: Occurs after 5 days of hospitalization, more likely involves multidrug-resistant organisms (Pseudomonas aeruginosa, Acinetobacter baumannii, MRSA) 2, 4
Post-Discharge Consideration
HAP can be diagnosed up to 14 days after hospital discharge if the infection was acquired during the hospitalization. 3, 5 Any infection appearing within 48 hours after discharge may be considered nosocomial in origin. 3
Clinical Significance
HAP is the second most common nosocomial infection after bloodstream infections and the most common infection in intensive care units. 2 The mortality rate is approximately 20%, with attributable mortality between 5-13%. 2 Healthcare costs range from $10,000 to $40,000 per case. 2