Does a Negative MRSA Test Exclude MRSA Infection?
A negative MRSA screening test does not completely exclude MRSA infection, but it makes MRSA infection highly unlikely in settings where MRSA prevalence is low to moderate (below 30-40%), with negative predictive values consistently exceeding 90-99%. 1, 2
Diagnostic Performance of Negative MRSA Screening
The utility of a negative MRSA test depends critically on your local MRSA prevalence:
High Negative Predictive Value in Low-Moderate Prevalence Settings
When MRSA prevalence is below 15%, negative MRSA colonization screening achieves a negative predictive value exceeding 90% for most infection types including bacteremia, respiratory infections, and skin/soft tissue infections 1
For S. aureus bacteremia specifically, negative MRSA screening provides NPV exceeding 95% when MRSA represents less than 19-24% of all S. aureus bacteremias 2
In skin and soft tissue infections, negative MRSA nasal swabs demonstrate an NPV of 97.5%, with specificity of 71.9% 3
Sensitivity Limitations
The major caveat is sensitivity ranges only 54-81% across different infection types, meaning negative screens miss 19-46% of actual MRSA infections 1, 2. This occurs because:
- Patients can have MRSA infection at non-colonized sites 1
- Hospital-acquired MRSA infections may develop after admission screening (nearly 60% of infected patients were MRSA-free on admission in one surgical study) 4
- Colonization may be intermittent or at non-nasal sites 5
Clinical Application Algorithm
When to Trust a Negative MRSA Screen
Use negative MRSA screening to withhold or de-escalate empiric vancomycin in:
Stable, non-critically ill patients without severe sepsis 2, 5
Settings where local MRSA prevalence is <30% among S. aureus isolates 4, 1
Patients lacking additional MRSA risk factors including 4, 5:
- Prior MRSA infection/colonization within past year
- Recent prolonged hospitalization
- Long-term antibiotic use
- Chronic wounds or osteomyelitis
- Hemodialysis
- Injection drug use
When screening was performed within 30 days of infection onset (or accounting for any prior positive results extends utility) 2
When NOT to Rely on Negative Screening Alone
Empiric MRSA coverage remains indicated despite negative screening in:
- Critically ill or septic patients where treatment failure poses unacceptable mortality risk 4
- ICU patients with severe pneumonia or VAP, where S. aureus prevalence exceeds 30% and MRSA accounts for approximately 50% of S. aureus isolates 4
- High local MRSA prevalence (>30-50% of S. aureus isolates) 4
- Patients with documented MRSA history within the past year 4
Enhanced Prediction Rules
Combining negative MRSA screening with absence of clinical risk factors achieves even higher NPV (98-100%) compared to screening alone (96%) 5. A validated clinical prediction rule incorporating 8 risk factors plus negative nares surveillance achieved 93% sensitivity and 98% NPV, compared to 72% sensitivity for screening alone 5.
Practical Stewardship Impact
- Patients with negative MRSA swabs receive approximately 1 day less vancomycin therapy in skin/soft tissue infections 3
- This strategy can prevent unnecessary vancomycin use and associated nephrotoxicity in low-moderate MRSA prevalence settings 1, 2
- The approach is most cost-effective when MRSA colonization rates are low-moderate 1
Critical Pitfalls to Avoid
Never use negative screening alone in unstable/septic patients - the 5-20% false-negative rate is unacceptable when mortality is at stake 1, 2
Don't ignore timing - screening performed >30 days before infection has reduced predictive value 2
Account for nosocomial acquisition - negative admission screening doesn't exclude hospital-acquired MRSA developing during prolonged stays 4
Consider non-nasal colonization sites - some patients harbor MRSA at other body sites despite negative nasal screening 5