Nasal colonization and lower respiratory tract infections with methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus is a cause of lower respiratory tract infections, particularly health care- and ventilator-associated pneumonia. Although many health systems use nasal screening for this microorganism for infection control, correlation between nasal carriage of the organism and development of infections due to it is not clear.

Records of patients admitted to medical intensive care between January 1, 2011, and December 31, 2012, were reviewed retrospectively. Patients' data were included if the patients were 18 years or older, satisfied clinical criteria for pneumonia, and had both nasal swabbing and culturing of respiratory specimens within 24 hours of admission.

A total of 165 patients met the inclusion criteria. Most had either community-acquired or health care-associated pneumonia. Of the 28 patients with a nasal swab positive for methicillin-resistant S aureus, 8 (4.8%) also had respiratory tract cultures positive for the microorganism. Among the 165 patients, 2 (1.2%) had negative nasal swabs but positive respiratory cultures. Sensitivity and specificity of nasal colonization with methicillin-resistant S aureus for subsequent infection with the pathogen were 80% and 87.1%, respectively; positive and negative predictive values were 28.6% and 98.5%, respectively.

Nasal screening for methicillin-resistant S aureus may be a valuable tool for de-escalation of empiric therapy targeted to the organism, especially in patients admitted for severe community-acquired or health care-associated pneumonia. The high negative predictive value suggests that patients with a negative nasal swab most likely do not have a lower respiratory tract infection caused by the organism.