MRSA Diagnosis and Treatment
Diagnostic Approach
For suspected MRSA infections, obtain deep tissue cultures by biopsy or curettage after wound cleansing and debridement, avoiding superficial swabs which provide inaccurate results. 1
When to Obtain Cultures
- Always culture infected wounds before starting empiric antibiotics when possible 1
- Cultures may be unnecessary for mild infections in patients without recent antibiotic exposure 1
- Do not culture clinically uninfected wounds unless for epidemiologic purposes 1
Optimal Specimen Collection
- Cleanse and debride the wound thoroughly before obtaining specimens 1
- Obtain deep tissue by curettage (scraping with sterile scalpel or dermal curette) or biopsy from the ulcer base 1
- Aspirate any purulent secretions with sterile needle and syringe 1
- Send promptly for aerobic and anaerobic culture with Gram stain 1
- Avoid swab specimens, especially from inadequately debrided wounds 1
Rapid Detection Methods
- Chromogenic agar plates and real-time PCR can detect MRSA directly from nasal and wound swabs with faster turnaround than traditional culture (18-24 hours) 2
- Molecular methods can detect MRSA directly from blood cultures 2
- MRSA nares screening has a 96.5% negative predictive value for ruling out subsequent MRSA infection within 7 days, making it a powerful stewardship tool for avoiding unnecessary anti-MRSA therapy 3
Treatment Approach
When to Cover MRSA Empirically
Provide empiric MRSA coverage in three specific situations: 1
- Prior MRSA history: Patient had MRSA infection or colonization within the past year 1
- High local prevalence: Local MRSA rates exceed 50% for mild infections or 30% for moderate soft tissue infections 1
- Severe infection: The infection severity makes treatment failure while awaiting cultures unacceptably risky 1
Skin and Soft Tissue Infections
For uncomplicated MRSA skin infections, incision and drainage alone may be sufficient in healthy patients with small purulent lesions. 4
Oral Therapy Options
- Clindamycin 600 mg three times daily 5
- Linezolid 600 mg twice daily 5, 6
- Trimethoprim-sulfamethoxazole (TMP-SMX) or doxycycline (combined with a β-lactam if streptococcal coverage needed) 1, 4
Intravenous Therapy Options
- Vancomycin (first-line for hospitalized patients due to cost, efficacy, and safety) 4
- Daptomycin 6 mg/kg/dose IV once daily 1, 5
- Linezolid 600 mg IV twice daily 6, 4
Duration
- 7-14 days total based on clinical response 5
- Uncomplicated infections: 5-10 days 5
- Switch from IV to oral when afebrile, improving local signs, and tolerating oral intake 5
Bacteremia
For uncomplicated MRSA bacteremia, treat with vancomycin or daptomycin 6 mg/kg/dose IV once daily for at least 2 weeks. 1, 5, 7
Defining Uncomplicated Bacteremia
All of the following must be present: 1, 5, 7
- Endocarditis excluded
- No implanted prostheses
- Follow-up blood cultures (obtained 2-4 days after initial positive) are negative
- Defervescence within 72 hours of effective therapy
- No metastatic infection sites
Complicated Bacteremia
- 4-6 weeks of therapy depending on infection extent 1, 5
- Some experts recommend higher daptomycin doses of 8-10 mg/kg/dose IV once daily 1, 5
Critical Management Steps
- Obtain follow-up blood cultures 2-4 days after initial positive cultures to document clearance 1, 5
- Perform echocardiography on all adult patients with MRSA bacteremia to exclude endocarditis 1, 5
- Identify and eliminate/debride source of infection 1
Infective Endocarditis
Treat MRSA endocarditis with IV vancomycin or daptomycin 6 mg/kg/dose once daily for 6 weeks. 1, 5, 7
- Some experts recommend higher daptomycin doses of 8-10 mg/kg/dose IV once daily 1, 5
- Do not add gentamicin to vancomycin for native valve endocarditis 1
- Do not add rifampin to vancomycin for native valve endocarditis 1
- Transesophageal echocardiography is recommended for all adult patients 1
Pneumonia
Never use daptomycin for MRSA pneumonia as it is inactivated by pulmonary surfactant. 8
- IV vancomycin or linezolid 600 mg twice daily 5
- Oral options: clindamycin 600 mg three times daily or linezolid 600 mg twice daily 5
- For empyema, antimicrobial therapy must be combined with drainage procedures 1, 5
Osteomyelitis
Treat MRSA osteomyelitis for a minimum of 8 weeks with surgical debridement. 5
Oral Options
- TMP-SMX 4 mg/kg/dose twice daily combined with rifampin 600 mg once daily 5
- Linezolid 600 mg twice daily 5
- Clindamycin 600 mg every 8 hours 5
Diabetic Foot Infections
Consider empiric MRSA coverage in diabetic foot infections with prior MRSA history, high local prevalence, or severe infection. 1
- Obtain bone specimen when MRSA osteomyelitis is suspected 1
- Linezolid showed 71% cure rate for diabetic foot infections with MRSA (versus 67% for comparator) 6
- Treatment duration: 1-2 weeks for mild infections, 2-3 weeks for moderate to severe 1
Prevention and Decolonization
Decolonization Indications
Consider decolonization only after optimizing wound care and hygiene measures fail to prevent recurrent SSTI or ongoing household transmission. 1
Decolonization Regimen
- Nasal mupirocin twice daily for 5-10 days 1
- Plus topical chlorhexidine for 5-14 days or dilute bleach baths 1
- Evaluate symptomatic household contacts and treat active infections before decolonization 1
Hygiene Measures
- Regular bathing and hand hygiene with soap/water or alcohol-based gel 1
- Avoid sharing personal items (razors, linens, towels) that contact infected skin 1
- Clean high-touch surfaces with commercially available cleaners 1
Critical Pitfalls to Avoid
- Never use β-lactam antibiotics alone for MRSA as they are completely ineffective 5
- Never use rifampin as monotherapy as resistance develops rapidly 1, 5
- Failure to drain abscesses leads to treatment failure regardless of antibiotic choice 5, 4
- Do not use daptomycin for pneumonia due to pulmonary surfactant inactivation 8
- Always obtain follow-up blood cultures 2-4 days after initial positive cultures in bacteremia 1, 5
- Always perform echocardiography in adult patients with MRSA bacteremia 1, 5
- Avoid swab cultures from inadequately debrided wounds 1