By Kathryn Radigan, MD, MSc
Assistant Professor,
Pulmonary Medicine,
Northwestern University,
Feinberg School of Medicine,
Chicago, IL

Dr. Radigan reports no financial relationships relevant to this field of study.

SYNOPSIS: The high negative predictive value of a negative nasal screen for methicillin-resistant Staphylococcus aureus suggests these patients do not have lower respiratory tract infections caused by the organism.

SOURCE: Tilahun B, et al. Nasal colonization and lower respiratory tract infections with methicillin-resistant Staphylococcus aureus. Am J Crit Care 2015;24:8-12.

Although nasal screening for methicillin-resistant Staphylococcus aureus (MRSA) is a widely accepted method for infection control, the relationship between nasal carriage and development of MRSA lower respiratory tract infection (LRTI) is not well studied. Tilahun and colleagues sought to determine the association between MRSA nasal swab results and MRSA LRTI in a medical ICU. In this single-site, retrospective cohort study, 165 patients were diagnosed with pneumonia and had both nasal swabbing and culturing of respiratory specimens within 24 hours of admission.

Among the 28 patients who had a nasal swab positive for MRSA, eight (4.8%) patients had respiratory specimens positive for MRSA. Of the 165 patients who were involved in the study, only two (1.2%) had negative nasal swabs but positive MRSA respiratory cultures. The sensitivity and specificity for nasal MRSA colonization for subsequent infection were 80% and 87.1% and the positive and negative predictive values were 28.6% and 98.5%, respectively.

COMMENTARY

It has long been recognized that treating our critically ill patients with early and appropriate antibiotics is a critical determinant of survival in septic shock.1 The Surviving Sepsis Campaign suggests that IV antimicrobials should be given within the first hour of recognition of septic shock and severe sepsis without septic shock.2 The guidelines for the choice of antibiotics are complicated and based on the individual patient and the identity and susceptibility pattern of the bacteria isolated on the individual unit. With good intention, antibiotics are often overprescribed with failure of timely de-escalation, leading to unintended adverse consequences including patient morbidity and mortality, increasing health care costs, and antimicrobial resistance. Prescribing antibiotics to cover MRSA empirically is one of the biggest culprits.3 This manuscript examined whether a correlation exists between MRSA nasal swab results and MRSA LRTI in a medical ICU. Even though researchers found that positive MRSA nasal swabs were not as helpful in guiding antibiotic therapy, they did conclude that the high predictive value of a negative nasal swab may be helpful with de-escalation of empiric antimicrobial therapy.

Although the overall message of this manuscript may be helpful, a word of caution should be exercised with these recommendations in regards to the type of specimen collected for respiratory tract culture. As described within the manuscript, only 5% of the specimens were collected by bronchoalveolar lavage; 65% of the specimens were tracheal aspirate or sputum collected by suctioning, 13% were from expectorated sputum, and 16% were collected from induced sputum. Since the majority of the specimens were not collected through bronchoalveolar lavage, the ability to differentiate upper respiratory tract colonization from lower tract pathogens is less accurate. Furthermore, the rate of pathogen detection for sputum samples can be quite poor, especially if standards of quality control for sputum culture are not followed. Taking into account these limitations, a negative nasal screen for MRSA should be an additional, but not the only, consideration to be included among a number of other important factors when deciding to de-escalate antibiotic therapy in our critically ill patients. 

REFERENCES

1. Kumar A, et al. Duration of hypotension before initiation of effective antimicrobial therapy is the critical determinant of survival in human septic shock. Crit Care Med 2006;34:1589-96.

2. Dellinger RP, et al. Surviving sepsis campaign: International guidelines for management of severe sepsis and septic shock: 2008. Inten Care Med 2008;34:17-60.

3. Rimawi RH, et al. Impact of regular collaboration between infectious diseases and critical care practitioners on antimicrobial utilization and patient outcome. Crit Care Med 2013;41:2099-2107.