From the Guidelines
Yes, a patient can have Methicillin-resistant Staphylococcus aureus (MRSA) pneumonia with a negative MRSA screen.
Key Points
- The sensitivity of MRSA screening is not 100%, which means that some cases of MRSA pneumonia may be missed by screening tests 1.
- A negative MRSA screen does not completely rule out the possibility of MRSA pneumonia, especially in patients with risk factors for MRSA infection, such as prior use of intravenous antibiotics 1.
- The diagnosis of MRSA pneumonia should be based on a combination of clinical presentation, laboratory results, and epidemiological factors, rather than relying solely on screening tests 1.
- Some studies have shown that MRSA colonization is associated with an increased likelihood of isolation of MRSA from respiratory samples, but this association is not strong enough to be used as a definitive diagnostic criterion 1.
- The use of Gram staining morphology can provide useful information for predicting MRSA pneumonia, but it is not a definitive diagnostic test and should be used in conjunction with other diagnostic tools 1.
Clinical Implications
- Clinicians should consider the possibility of MRSA pneumonia in patients with severe community-acquired pneumonia, even if the MRSA screen is negative 1.
- Empirical antibiotic treatment should be started early in patients with suspected MRSA pneumonia, and should be guided by local epidemiology and susceptibility patterns 1.
- The use of rapid diagnostic tests, such as PCR, can help to quickly identify MRSA pneumonia and guide antibiotic treatment 1.
From the Research
MRSA Pneumonia with Negative MRSA Screen
- A patient can have Methicillin-resistant Staphylococcus aureus (MRSA) pneumonia even with a negative MRSA screen, as evidenced by a case report where a patient with a negative MRSA nasal swab was later diagnosed with MRSA pneumonia by endotracheal aspirate culture (EAC) 2.
- The sensitivity of MRSA nasal swab screening can be low, ranging from 50.0% to 68.5%, which means that a negative result does not completely rule out MRSA pneumonia 3, 4.
- However, the negative predictive value of MRSA nasal swab screening can be high, ranging from 92.1% to 100%, which means that a negative result can be used to guide antibiotic de-escalation in patients with low risk of MRSA pneumonia 5, 4.
Clinical Implications
- In critically ill patients with high risk and suspicion for MRSA pneumonia, discontinuing empiric MRSA coverage based on a negative MRSA nasal swab should be done with caution, and clinicians should wait until respiratory culture results are obtained before de-escalating antibiotics 2, 3.
- MRSA nasal swab screening can be a useful tool for antimicrobial stewardship, allowing for the early de-escalation of MRSA coverage in hospitalized patients requiring empiric antibiotics 4, 6.
- The use of MRSA nasal PCR testing protocols, such as those led by pharmacists, can reduce vancomycin days of therapy, average length of stay, and 30-day readmission rates in patients with pneumonia 6.