Methicillin-resistant Staphylococcus aureus (MRSA) nasal screening has demonstrated effectiveness in reducing the duration of vancomycin therapy for lower respiratory tract infections; however, its applicability to other types of infections remains unclear. Due to this uncertainty, a 2018 literature review aimed to evaluate the utility of MRSA nasal screening beyond lower respiratory tract infections. Regarding nonspecific (any source) infections, a retrospective review of 273 patients assessed MRSA nasal screening alongside blood, wound, or respiratory cultures within 48 hours of hospital admission, showing sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of 58.3%, 93.9%, 30.4%, and 98.0%, respectively. In another retrospective study of 11,882 intensive care unit patients, only 0.22% of those with negative MRSA nasal swab results developed clinically significant MRSA infections, resulting in a high NPV of 99.4%. Notably, it was estimated that 7,364 days of vancomycin therapy could have been avoided if vancomycin had been discontinued in patients with negative swab results. [1]
For bloodstream infections, a retrospective review of patients with S. aureus bacteremia (N= 409) found that MRSA swab results obtained within the previous 30 days had NPV greater than 95% and 90% if the proportion of MRSA bloodstream isolates was less than 19.3% and 33.5%, respectively. Another retrospective cohort study of MRSA screening in patients with S. aureus bacteremia (N= 799) showed a sensitivity of 56%, specificity of 98%, PPV of 88%, and NPV of 91%. Although it was concluded that a negative MRSA nasal screening was not accurate enough to rule out MRSA bacteremia, it was suggested that a positive MRSA nasal swab should prompt consideration for initiation of anti-MRSA therapy. Subgroup analyses of larger studies also showed varying performance of MRSA nasal screening in predicting MRSA bloodstream infections, indicating inconsistencies in effectiveness. The authors suggest that empiric vancomycin may be necessary regardless of nasal screening, especially if there's a risk of bloodstream infection from other methicillin-resistant gram-positive bacteria. A positive MRSA nasal screen might prompt consideration for anti-MRSA therapy in high-risk patients, despite typically low PPVs. While routine incorporation of MRSA nasal screening in managing S. aureus bacteremia seems unlikely, it could be useful in stable patients without risk factors for methicillin-resistant pathogens. [1]
Notably, guidelines recommend empiric vancomycin for patients with healthcare-associated intra-abdominal infections known to be colonized with MRSA or at risk due to prior treatment failure and significant antibiotic exposure, while advising against empiric anti-MRSA therapy for patients with community-acquired intra-abdominal infections. Limited evidence exists on the performance of MRSA nasal screening in predicting intra-abdominal infections, with studies reporting varying sensitivities and specificities. Despite this, MRSA nasal screening results provide additional data to consider when deciding to initiate or de-escalate anti-MRSA therapy, potentially leading to improved patient outcomes and cost savings. The implementation of universal MRSA nasal screening prior to major gastrointestinal surgery could potentially prevent surgical site infections and yield significant cost savings. [1]
Regarding skin and soft tissue infections (SSTI) and bone and joint infections, retrospective studies have shown varying performances of MRSA nasal screening. For SSTI, one study reported a sensitivity of 60.0%, a specificity of 71.8%, a PPV of 6.6%, and an NPV of 98.1%. Conversely, in diabetic foot infections, MRSA nasal screening demonstrated a sensitivity of 41%, specificity of 90%, PPV of 67%, and NPV of 80%. Additionally, a retrospective evaluation of MRSA screening in 102 patients with prosthetic joint infections secondary to Staphylococcus spp. reported overall poor performance, with neither sensitivity nor NPV exceeding 80%. However, the prevalence of MRSA infection in this study was high (37%) compared to other evaluated studies. These findings suggest challenges in predicting MRSA infection, particularly in contexts with high MRSA prevalence. Importantly, the authors emphasize that prospective data on using MRSA nasal screening for de-escalation of empiric therapy in SSTI are currently lacking. [1]
At the time of this review's publication, no independent studies assessed the efficacy of MRSA nasal screening for predicting MRSA urinary tract infections (UTI). However, two large retrospective reviews conducted subgroup analyses for UTI patients, reporting sensitivities ranging from 71.0% to 77%, specificities from 79.3% to 87%, PPV from 11% to 13.3%, and NPV exceeding 98%. Although MRSA nasal screening shows promise as a tool for guiding empiric antibiotic selection and de-escalation in UTIs, MRSA is uncommon in urinary pathogens, with a prevalence of less than 5%, and empiric anti-MRSA therapy is rarely warranted in this context. As for central nervous system infections, the performance of MRSA nasal screening remains unexplored, raising concerns about its safety in guiding antibiotic selection for high-risk meningitis patients. Overall, the authors suggest that MRSA nasal screening serves as a valuable antimicrobial stewardship tool, extending its potential applications beyond lower respiratory tract infections. Negative MRSA nasal screening in appropriately selected patients can prevent unnecessary initiation or guide discontinuation of anti-MRSA therapy. However, further investigation of MRSA nasal screening's clinical utility for non-respiratory tract infections is warranted as its usage becomes more widespread. [1]
Similarly, another review article published in 2022 discusses the role of MRSA nasal swabs in guiding empiric antibiotic therapy for various infections. The predictive value of MRSA nasal screening varies by clinical syndrome and patient population. For skin and soft tissue infections, especially purulent infections in areas with high MRSA prevalence, a negative nasal swab does not reliably rule out MRSA, and empiric anti-MRSA therapy remains appropriate. However, in hospitalized patients with diabetic foot infections or nonpurulent cellulitis, a negative swab may support de-escalation of MRSA-targeted therapy. For intra-abdominal infections, MRSA is an uncommon pathogen in community-acquired cases, and screening is generally not indicated. In health care–associated cases, the high NPV of a nasal swab may help avoid or de-escalate anti-MRSA therapy. Similarly, for urinary tract infections, where MRSA is rarely implicated, MRSA screening offers limited benefit. Among immunocompromised patients, such as those with leukemia or post-transplant, data are limited, but high NPVs suggest a possible role for MRSA screening in de-escalating therapy in selected situations. Importantly, in critically ill patients with septic shock or suspected bloodstream infections, empiric antibiotic decisions should not rely on MRSA nasal screening results alone. Overall, MRSA nasal screening is best used to rule out MRSA infection due to its high NPV across various clinical scenarios, while a positive swab should not be used alone to justify anti-MRSA therapy. [2]
A 2018 systematic review and meta-analysis further investigated the potential role of MRSA colonization in determining the need for empiric coverage. A total of 29 studies were included, with 24,225 patients. Results revealed that in infectious cases where the pathogen was either not known or included organisms other than Staphylococcus aureus, pooled specificities for bacteremia, lower respiratory tract infections, and skin and soft tissue infections (SSTI) were greater than 85%. For SSTIs specifically, sensitivity was 54.0% (95% confidence interval [CI] 37.7 to 69.4) and specificity was 92.9% (95% CI 90.7 to 94.5). In most instances where the prevalence of MRSA as a causative organism is <15%, the negative NPV of a negative MRSA colonization swab was found to be > 90%. [3]
Lastly, a 2023 single-center, before-and-after study examined vancomycin overuse between May 2020 and June 2021, aiming to investigate the effectiveness of MRSA nasal screenings for de-escalating vancomycin therapy in patients suspected of MRSA infections. The study included adult patients with suspected MRSA infections such as pneumonia, intra-abdominal infections, bacteremia, and skin infections, while excluding urinary tract infections. Each patient undergoing the intervention was swabbed in the nares prior to vancomycin administration. Vancomycin therapy days were measured before and after the intervention including the number of swabs collected after the implementation of the intervention. The intervention led to a significant increase in the use of vancomycin nares swabs (28/150 vs. 48/100, p= 0.040) immediately pre/post-intervention, and a notable decrease in vancomycin usage days per 1,000 patient days by 2.34% per month (p= 0.0039) over a two-year period post-intervention, demonstrating the efficacy of MRSA nasal screenings in reducing unnecessary vancomycin therapy in various types of infections outside the respiratory tract. [4]