Pneumonia is defined in the Infectious Diseases Society of America / American Thoracic Society (IDSA / ATS) guidelines as the presence of a new lung infiltrate with clinical evidence indicating the infiltrate is infectious in origin. Clinical signs of an infectious lung process include fever, production of sputum (which is often purulent), leukocytosis, and hypoxia. While this definition is broad, it is not exhaustive, and it should be acknowledged that there is no gold standard for the diagnosis of hospital-acquired pneumonia (HAP).

One additional consideration that distinguishes HAP from community-acquired pneumonia (CAP) is that HAP is defined as an infection not incubating at the time of hospitalization and occurring 48 hours or longer after hospital admission. This distinction has implications on the types of pathogens that are suspected as etiologic agents of infection, the appropriate modes of diagnosis, and the appropriate empiric antimicrobial regimens chosen for treatment.

The incidence of HAP is approximately 5-10 cases per 1000 hospital admissions; this is the most common cause of hospital-acquired infection in the United States and Europe.

By and large, HAPs are commonly due to pathogens such as Pseudomonas aeruginosa and other gram-negative bacilli (Escherichia coli, Klebsiella pneumoniae, Enterobacter spp, etc) and gram-positive cocci such as methicillin-resistant Staphylococcus aureus (MRSA) when compared to CAPs. Not all patients are at equal risk for multidrug-resistant pathogens, however, and the primary risk factor associated in the literature with the development of multidrug-resistant HAP is prior exposure to intravenous (IV) antibiotics in the preceding 90 days. With respect specifically to MRSA pneumonia, there is some evidence that supports that positive MRSA screening from nasal samples increases the risk of MRSA pneumonia, although the evidence is not strong enough for this to be considered a risk factor. Evolving data supports a strong negative predictive value of a screening MRSA nasal swab polymerase chain reaction (PCR) assay; in studies, the negative predictive value has been reported as high as 97%-99.2%. Risk factors for infection with Pseudomonas also include a history of chronic obstructive pulmonary disease, structural lung disease, and use of mechanical ventilation.

HAP is often diagnosed by chest radiograph prompted by clinical symptoms and signs of a respiratory infection. Patients may have noted dyspnea and sputum production on examination, as well as focal or diffuse lung findings of rhonchi or crackles on auscultation, with the occasional finding of egophony indicating lung consolidation. While sputum culture is generally beneficial to guide and narrow empiric antibiotic therapy, it is often difficult to obtain and often limited by oropharyngeal contamination.

Multiple clinical prediction tools exist that have been used in the evaluation and management of CAP (ie, CURB65 and Pneumonia Severity Index) and ventilator-associated pneumonia (such as the Clinical Pulmonary Infection Score), but no specific scores exist for HAP. IDSA guidelines do not support the use of the Clinical Pulmonary Infection Score to determine if patients suspected of having HAP should be treated with empiric antibiotics.

Complications of HAP are common, occurring in 50% of cases. These include, but are not limited to, development of sepsis or septic shock, renal failure, and respiratory failure with the potential to progress to need for mechanical ventilation and possible development of acute respiratory distress syndrome (ARDS). HAP carries the highest mortality rate of all nosocomial infections, with a directly attributable mortality of 14% for non-intensive care unit (ICU) HAP and an overall mortality rate of 20%. Mortality rates vary greatly depending on the patient substrate and location, ranging from 7% on general wards to as high as 62% in bone marrow transplant units.

Note: To view x-rays and other imagery, see individual diagnoses.

Related topic: Haemophilus influenzae pneumonia