5 Things To Know About Interpreting MRSA Nares Swab Results

In this article two infectious diseases pharmacists discuss five things to know about interpreting MRSA nares swab results

Authored By: Hunter Rondeau, Pharm.D., BCIDP & Timothy P. Gauthier, Pharm.D., BCPS, BCIDP

Article Posted 16 April 2025

The methicillin-resistant Staphylococcus aureus (MRSA) nares swab (also referred to as a MRSA nasal swab) is a valuable tool for infection control programs, helping to identify which patients are colonized with MRSA and in turn may be at risk for MRSA infection. For antimicrobial stewardship purposes, MRSA nares screening can be particularly helpful for antibiotic de-escalation. This is because a test indicating MRSA is not present, can help rule it out as a potential pathogen, and thus enable streamlining of the antibiotic regimen.

Proper MRSA nares result interpretation is crucial to avoid inappropriate decision making. This includes antibiotic initiation, continuation and discontinuation. The widespread overuse of vancomycin is a serious public health concern, primarily driven by its use for less severe infections where alternative antibiotics would be effective. This practice contributes to the rise of vancomycin-resistant Enterococci (VRE), a resistant organism with remarkably few therapies. Inappropriate vancomycin use also puts patients at risk for toxicities, such as renal failure.

Given the value of MRSA nares as a tool to avoid unnecessary anti-MRSA therapy in patients hospitalized with pneumonia, it has garnered significant popularity over the last decade. To help promote this tool, we will touch upon 5 things to know about MRSA nares results…

1. Negative MRSA nares should trigger consideration for antibiotic regimen adjustment

The primary strength of the MRSA nasal swab lies in its high negative predictive value (NPV) in certain clinical scenarios. A negative result, especially in pneumonia, can reliably help identify patients unlikely to have MRSA infection. Thus allowing for safe discontinuation of agents targeting MRSA such as vancomycin or linezolid. Remember, “a negative MRSA nares = vancomycin is no longer needed” is a common, and often appropriate, interpretation in the proper clinical context.

It is notable that a negative result in the computer DOES NOT mean the patient is not at risk for MRSA. Be careful not to assume this without reviewing the other considerations noted below.

2. Recent exposure to mupirocin or other decolonizing agents may result in a false-negative test

If a patient is admitted to a hospital and goes to an ICU with a central venous catheter, it is common for a universal MRSA decolonization protocol to be initiated. This means the patient is bathed daily with chlorhexidine and mupirocin is applied to the nares twice daily for 5-10 days. With universal decolonization rather than swabbing and waiting for a result to decolonize or not, patients are all decolonized as a standard practice. This removes the delay in getting the swab result and removes the costs associated with performing swabs. It does lead to unnecessary use of mupirocin and that may negatively impact mupirocin resistance rates, but if used strategically these risks can be reduced.

Imagine a patient presents to the hospital demonstrating symptoms of pneumonia and has risk factors for MRSA. An MRSA nares swab is ordered but for some reason is never collected. The patient receives MRSA decolonization protocol for 48 hours and then the MRSA nares swab is finally collected. This is a setting where a false-negative may be found. Thus clinicians must not assume a negative test in the computer means the patient is not colonized.

Prevalence matters for interpreting test results, including MRSA nares results

Prevalence going up makes NPV go down. For NPV, increasing prevalence decreases the numerator while increasing the denominator, leading to an overall decrease in in NPV:

NPV = (specificity x (1 – prevalence)) / [((1 – sensitivity) x prevalence) + (specificity x (1 – prevalence))]

Prevalence going up makes PPV go up. For PPV, increasing prevalence increases the numerator while decreasing the denominator, leading to an overall increase in PPV:

PPV = (sensitivity x prevalence) / [(sensitivity x prevalence) + ((1 – specificity) x (1 – prevalence))]

The other components of these equations are the specificity and sensitivity. These factors can be influenced by things such as which infectious syndrome is under concern and how a specimen is collected. This is why we are focusing on pneumonia in this post, because that is where the MRSA nares has the greatest NPV data.

The take home point here is to try to understand your local MRSA prevalence rates so you can get a feel for how it impacts the local NPV and PPV of MRSA nares results.

3. Positive MRSA nares results are not very helpful for diagnosing an infection

While the NPV for MRSA nares is quite good for pneumonia, the PPV value is not great pneumonia.

Remember that any positive result from the lab should not = knee jerk start antibiotics. The appropriate immediate response is to evaluate the clinical status of the patient for current signs and symptoms of infection. Staphylococcus aureus is a bacteria notorious for growing in the lab when an infection is present. The notion that other tests would be negative yet the MRSA nares is positive seems unlikely in the setting of an invasive MRSA infection.

4. Opportunity to utilize MRSA nares results varies by disease state

When a patient presents with an infection, a clinician can evaluate the type of infection they have in the context of the patient’s past medical history, which enables the clinician to differentiate likely causative organisms.

For example, if someone had an MRSA infection of the skin a month ago and is back again with a skin infection, it is reasonable to first consider it may be MRSA again. Alternately, if someone had their first ever urinary tract infection a month ago due to E. coli and are presenting again with urinary symptoms, it is pretty unlikely an organism like MRSA is the cause. It does not make much sense to go chasing unicorns; it is not best practice to go covering for organisms when a patient lacks risk factors them and they are uncommon for that infection type.

Over treatment = unnecessary increased risk of consequences. With antibiotics those consequence include toxicity, antimicrobial resistance, and wasted resources. If MRSA is unlikely at baseline, perhaps do not order an MRSA nares at all if the result is unlikely to change management.

5. Do not get overzealous with interpreting MRSA nares results

Given it is so common that an negative MRSA nares result allows for discontinuation of MRSA therapy with pneumonia, over time it can be easy to grow overly comfortable with seeing the result in the computer and immediately thinking it is fine to change therapy.

For one, sometimes clinicians can be resistant to the intervention. Pushing too hard on the prescriber can damage important relationships. Beware accidentally trying to win the battle at expense of losing a key partner to fight with in the war against antibiotic resistant microbes.

Additionally, there are circumstances in which continuing treatment is reasonable, despite a reliable negative MRSA nares result. This may be a debated topic and nuanced, but mechanically ventilated patients, immunocompromised patients and severely ill patients at high risk for mortality are groups that should prompt additional caution.

DISCLAIMER: This article was written with the assistance of Google Gemini (artificial intelligence software). The views and opinions in this article represent those of the authors and do not necessarily reflect the policy or position of any past, current, or potential future employer.

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