Meta-Analysis On the Utility of Nasal Swabs to Rule Out MRSA Pneumonia

Methicillin-resistant Staphylococcus aureus (MRSA) is an increasingly common and potentially deadly pathogen seen in cases of hospital-acquired pneumonia (HAP) and community-acquired pneumonia (CAP). Risk factors for invasive S. aureus infection include nasal colonization.^1,2 This can carry significant clinical impact given that nasal colonization of S. aureus occurs in about 25%-30% of the population, with varying proportions of methicillin-resistant isolates.^3,4 As a result, many patients presenting with pneumonia are empirically treated with anti-MRSA therapy, leaving antimicrobial stewardship programs (ASPs) faced with the challenge of identifying patients who truly warrant continuation of anti-MRSA therapy.

Although the role of MRSA nasal swabs to rule out MRSA pneumonia has been of particular interest to a number of researchers, Dr. Parente and colleagues are the first to consolidate the available data describing the predictive value of the nares screen. This recently published meta-analysis included 22 relevant studies that described the relationship between rates of positive MRSA nasal swabs and diagnosed MRSA pneumonia.⁵ The primary outcome was diagnostic performance described through pooled sensitivity, specificity, positive predictive values (PPV), negative predictive values (NPV), likelihood ratios, and diagnostic odds ratios.

Most of the included studies were retrospective in nature (18/22, 81.8%) and performed at teaching hospitals (16/22, 72.7%), and of those reporting types, ventilator-associated pneumonia (VAP, 8/11, 72.7%) predominated. Diagnostic criteria and MRSA screening processes were similar across all reported studies.

The diagnostic performances of MRSA nasal swabs across different classifications of pneumonia are reported in Table 1. Overall, MRSA nasal swabs demonstrate a 96.5% NPV, 90.3% specificity, and 0.32 negative likelihood ratio in the combined analysis. Of note, the low sensitivity rate in patients with VAP suggests that the benefit of the MRSA nasal swabs may not be applicable in this population due to iatrogenic sources of MRSA from intubation. As reflected by its relatively low PPV, positive MRSA nasal swabs cannot distinguish between colonization versus true infection. The authors concluded that nares screening can prove valuable in ruling out MRSA pneumonia, especially in CAP and HCAP, and aiding in de-escalation off of empiric antibiotics.

Streamlining empiric antibiotics continues to be a challenge for clinicians, particularly in the absence of microbiologic data. This meta-analysis further drives home the value of MRSA nasal swabs for ASPs to quickly and safely de-escalate patients off anti-MRSA therapy while curbing rates of side effects, need for lab monitoring, and costs. Although the findings of this study question their utility in patients with VAP, MRSA nasal swabs can be leveraged as a powerful de-escalation tool in patients presenting with CAP and HCAP. With the next iteration of the CAP guidelines soon to be released, it is anticipated that the rapidly growing body of literature supporting the MRSA nasal swab will be incorporated and its use will be encouraged.

References:

1. Kluytmans J, van Belkum A, Verburgh H. Nasal carriage of Staphylococcus aureus: epidemiology, underlying mechanisms, and associated risks. Clin Microbiol Rev. 1997 Jul;10(3):505-520.
2. Von Eiff C, Becker K, Machka A, Peters G. Nasal carriage of Staphylococcus aureus bacteremia. N Engl J Med. 2001 Jan 4:344(1):11-16.
3. Kuehnert MJ, Kruszon-Moran D, Hill HA, et al. Prevalence of Staphyloccocus aureus nasal colonization in the United States, 2001-2002. J Infect Dis. 2006;193(2):172-179.
4. Honda H, Krauss MJ, Coopersmith CM, et al. Staphylococcus aureus nasal colonization and subsequent infection in intensive care unit patients: dose methicillin resistance matter? Infect Control Hosp Epidemiol. 2010;31(6):584-591. doi: 10.1086/652530.
5. Parente D, Cunha C, Mylonakis E, Timbrook TT. The clinical utility of methicillin resistant Staphylococcus aureus (MRSA) nasal screening to rule out MRSA pneumonia: a diagnostic meta-analysis with antimicrobial stewardship implications. Clin Infect Dis. 2018. doi: 10.1093/cid/ciy024