In this episode, I’ll discuss how well a negative nasal MRSA screen rules out infection with MRSA.
Many hospitalized patients are treated empirically with anti-MRSA antibiotics only to have cultures come back negative for MRSA after several days of anti-MRSA therapy. This represents an antibiotic stewardship opportunity if the cost and adverse effects of these antibiotics could be avoided while still providing treatment to patients with an MRSA infection.
To quantify the negative predictive value of a nasal MRSA screen, a retrospective cohort across VA medical center hospitals was analyzed and published in the journal Clinical Infectious Diseases in October 2019.
After the first collection of an MRSA nasal screen, clinical cultures for the next 7 days were analyzed in a cohort over half a million patients.
A nasal MRSA screen was found to have a sensitivity of 67% and a specificity of 81%.
Determining the usefulness of a test does not stop with specificity and sensitivity. Predictive values must also be considered. The positive predictive value of a test describes the likelihood of the patient actually having the disease given a positive result. A negative predictive value of a test describes the likelihood of the patient not having the disease given a negative result.
In this study the nasal MRSA screen had a positive predictive value of 24.6% and a highly clinically relevant negative predictive value of 96.5%.
The negative predictive value of a nasal MRSA screen remained >96% for ruling out MRSA bacteremia, catheter infection, intra-abdominal infection, respiratory infection, and urinary infection. Only wound infections had a slightly lower NPV of about 93%.
Previous small and usually single-center studies have suggested that nasal MRSA screens have an excellent negative predictive value, and this larger multi-center cohort confirms those findings.
A major limitation of this study is the fact that the outcome studied was not MRSA infection, rather it was the surrogate measure of an MRSA positive clinical culture.
In the discussion section, the authors state that the nasal MRSA screen cannot be used to de-escalate antibiotics in critically ill patients, however there is no mention of excluding the critically ill elsewhere in the study. The current status of the study is an ‘accepted manuscript’ which means that it has not yet been copyedited or formatted per to journal style, so this may become more clear in the final published version.
It should also be noted that due to the very low positive predictive value found in this study that a positive nasal MRSA screen cannot adequately predict positive clinical cultures with MRSA.
Despite these limitations, this study greatly clarifies and supports the role of a nasal MRSA screen in identifying a cohort of patients that are unlikely to need further treatment with anti-MRSA antibiotics. The nasal screen is available in a few hours compared to waiting a few days for culture results and has the potential to assist antimicrobial stewardship efforts.
To download a copy of my free visual critical care antibiotic coverage guide, go to pharmacyjoe.com/abx.
If you like this post, check out my book – A Pharmacist’s Guide to Inpatient Medical Emergencies: How to respond to code blue, rapid response calls, and other medical emergencies.