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 deescalation off of empiric antibiotics.