Does a Positive Methicillin-Resistant Staphylococcus aureus (MRSA) Nasal Screen Predict the Risk for MRSA Skin and Soft Tissue Infection?

Abstract

Background: Skin and soft tissue infections (SSTIs) are often caused by gram-positive bacteria that colonize the skin. Given the overuse of antibiotics, SSTIs are increasingly caused by resistant bacteria, including methicillin-resistant Staphylococcus aureus (MRSA). Guidance on the utility of MRSA nasal screening for MRSA SSTI is limited. Objective: To determine whether MRSA nasal screening predicts the risk of MRSA SSTIs. Methods: This was a single-center, retrospective cohort study of adult patients with an SSTI diagnosis that had MRSA nasal screening and wound cultures obtained within 48 hours of starting antibiotics. Sensitivity, specificity, positive and negative predictive value, and positive and negative likelihood ratios were calculated using VassarStats. Pretest and posttest probabilities were estimated with Microsoft Excel. Results: A total of 884 patient encounters were reviewed between December 1, 2018, and October 31, 2021, and 300 patient encounters were included. The prevalence of MRSA SSTI was 18.3%. The MRSA nasal colonization had a sensitivity of 63.6%, specificity of 93.9%, positive predictive value of 70.0% (95% CI = 55.2%-81.7%), negative predictive value of 92.0% (95% CI = 87.7%-94.9%), positive likelihood ratio of 10.39 (95% CI = 6.12-17.65), negative likelihood ratio of 0.39 (95% CI = 0.27-0.55), positive posttest probability of 70.0%, and negative posttest probability of 8.0%. Conclusions: Given the high positive likelihood ratio, a positive MRSA nasal screen was associated with a large increase in the probability of MRSA SSTI at our institution, and a negative MRSA nasal screen was associated with a small but potentially significant decrease in the probability of MRSA SSTI.

Current challenges in acute bacterial skin infection management

“…Whether we can reliably predict the absence of MRSA in SSTI has yet to be established with certainty. Data suggest that nasal carrier state negativity has an NPV of 75–92% at exclusion [27 ▪▪ ,28 ▪ ]. We have not found strong evidence that a negative nasal carrier status has the same efficacy in ruling out MRSA SSTI as for lower respiratory tract infections.…”

Rapid diagnostics for skin and soft tissue infections: the current landscape and future potential

“…A part from microbiological data obtained through sampling of the infected tissue, it should be kept in mind that the lack of culture-based detection of nasal carriage of MRSA has a negative predictive value >90% to rule out this pathogen in HA-SSTI [68,69]. The use of PCR assays for a faster screening of nasal carriage could help targeting patients with SSTI due to MRSA.…”

Skin and soft tissue infections in solid organ transplants

“…Methicillin-resistant Staphylococcus aureus (MRSA) nasal screening, bacterial and fungal blood cultures, serum biomarkers (e.g., galactomannan and 1-3-β- d -glucan), PCR in blood specimens (e.g., CMV DNA), antigen tests (e.g., urinary histoplasma antigen and Streptococcus pyogenes pharyngeal screening) may support the diagnosis [13]. Additional examinations for systemic involvement are also important and, depending on the pathogen, may include radiographic diagnostic tests and performance of biopsies in anatomic sites different from skin.…”