How to effectively rule out Methicillin-resistant Staphylococcus aureus (MRSA) infection?

From the Guidelines

To effectively rule out MRSA infection, obtain appropriate cultures before starting antibiotics, and consider empiric treatment with antibiotics such as trimethoprim-sulfamethoxazole, doxycycline, clindamycin, or linezolid, while awaiting culture results. The gold standard for diagnosing MRSA infection is collecting samples from the suspected infection site, such as wound swabs, blood cultures, or tissue specimens, and sending them for bacterial culture and susceptibility testing 1. Results typically take 48-72 hours, during which empiric antibiotic therapy may be necessary. Rapid diagnostic tests like PCR-based assays can provide results in hours rather than days, though they're not available everywhere. Nasal swabs can help identify MRSA colonization but don't confirm active infection. For skin and soft tissue infections, incision and drainage remains the primary treatment, with culture of the purulent material.

Key Considerations

• Obtain appropriate cultures before starting antibiotics
• Consider empiric treatment with antibiotics such as trimethoprim-sulfamethoxazole, doxycycline, clindamycin, or linezolid, while awaiting culture results
• Incision and drainage is the primary treatment for skin and soft tissue infections, with culture of the purulent material
• Definitive exclusion of MRSA requires negative culture results or identification of a different causative organism, along with clinical improvement on appropriate therapy

Empiric Treatment Options

• Trimethoprim-sulfamethoxazole (1-2 DS tablets twice daily)
• Doxycycline (100mg twice daily)
• Clindamycin (300-450mg four times daily)
• Linezolid (600mg twice daily)

Severe Infections

• Intravenous options include vancomycin, daptomycin, or linezolid It's essential to note that the management of MRSA infections should include identification, elimination, and/or debridement of the primary source and other sites of infection when possible, as recommended by the Infectious Diseases Society of America 1. Additionally, follow-up blood cultures 2–4 days after initial positive cultures and as needed thereafter are recommended to document clearance of bacteremia 1.

From the FDA Drug Label

Persisting or relapsing S. aureus bacteremia/endocarditis: Perform susceptibility testing and rule out sequestered foci of infection. ( 5.9)

To effectively rule out MRSA infection, perform susceptibility testing and rule out sequestered foci of infection 2. This is a crucial step in managing patients with persisting or relapsing S. aureus bacteremia/endocarditis. Key considerations include:

• Susceptibility testing: to determine the effectiveness of antibiotics against the specific strain of S. aureus
• Rule out sequestered foci of infection: to identify and treat any potential sources of infection that may be contributing to the persistence or relapse of the infection.

From the Research

Effective Methods to Rule Out MRSA Infection

To effectively rule out MRSA infection, several methods can be employed:

• MRSA polymerase chain reaction (PCR) nasal screening can be used as a tool for antimicrobial stewardship, with a high negative predictive value for MRSA infections 3
• This method can help limit unnecessary antibiotics for any infectious source and decrease antibiotic-associated morbidity, resistance, and costs 3
• The use of MRSA PCR nasal swabs has been shown to have a negative predictive value of 100% for MRSA urinary tract infection, 97.9% for MRSA bacteremia, 97.8% for MRSA pneumonia, 92.1% for MRSA wound infection, and 96.6% for other MRSA infections 3

Treatment Options for MRSA Infections

For patients with suspected MRSA infections, various treatment options are available:

• Vancomycin is a standard empiric choice for severe/invasive infections, but alternative therapies such as linezolid, daptomycin, and ceftaroline can be used 4, 5
• Linezolid has been shown to be effective against MRSA bacteremia, with comparable effectiveness and safety to vancomycin, teicoplanin, or daptomycin 6
• The choice of antibiotic should be based on the site and severity of the infection, as well as the patient's medical history and potential drug interactions 4, 7

Considerations for Antibiotic Use

When using antibiotics to treat MRSA infections, several considerations should be kept in mind:

• The length of antibiotic treatment required depends on the severity of infection and pre-existing co-morbidities 7
• Monitoring response to treatment is important to ensure cure of infection whilst preventing excessive antibiotic use 7
• Debridement and drainage, in addition to prosthesis removal, may be necessary in some cases 7