Nasal Colonization and Lower Respiratory Tract Infections with Methicillin-Resistant Staphylococcus aureus

Abstract

Background 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. Methods 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. Results 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 methicillinresistant 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 methicillinresistant 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. Conclusions 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.

Nasal-Swab Results for Methicillin-Resistant Staphylococcus aureus and Associated Infections

“…Unfortunately, there were too few individual endotracheal tube aspirate, sputum, and BAL samples to analyze. However, studies including respiratory samples 15,17,18 have reported sensitivity and specificity similar to those found in the current study.…”

“…Reported values for sensitivity and specificity of MRSA nasal-swab screening alone for predicting MRSA infection have ranged from 50.0% to 88.0% and from 83.0% to 100.0%, respectively. [15][16][17][18][19][20] The comparability of these results is limited by the different nasal- swab assays used at each site. Many studies used polymerase chain reaction, which has been shown to increase sensitivity 22 relative to the chromogenic medium used at the current study site.…”

“…The required sample size was calculated using precision calculations based on different sensitivity and specificity estimates found in the current literature, [15][16][17][18][19][20] and the largest sample size was chosen to ensure adequate precision for the study. The precision calculation was based on a 95% confidence interval (CI), an infinite population size, and a precision of 5%.…”

“…[15][16][17][18][19][20] The populations studied have included patients with various types of pneumonia, skin and soft-tissue infections, or multiple sites of potential infection, as well as patients admitted to the intensive care unit. [15][16][17][18][19][20] Furthermore, the current literature [15][16][17][18][19][20] focuses on patients with documented MRSA infection or patients at high risk for nosocomial infection, with patients being followed to determine whether they developed MRSA infection. To our knowledge, no studies have examined consecutive patients presenting to hospital who were started on empiric IV vancomycin therapy.…”

Nasal Methicillin-Resistant Staphylococcus aureus (MRSA) PCR Testing Reduces the Duration of MRSA-Targeted Therapy in Patients with Suspected MRSA Pneumonia

“…These studies have consistently shown that MRSA nasal colonization has a high negative predictive value (Ͼ94%) for MRSA recovered from lower respiratory sources, suggesting that the absence of MRSA in the nares can be used to predict the absence of MRSA in lower respiratory tract cultures (10)(11)(12)(13). A prior retrospective study from our group demonstrated that MRSA nasal culture screening had a 98.5% negative predictive value for MRSA lower respiratory tract cultures.…”