MRSA Screening Test
The recommended screening test for MRSA colonization is a nasal swab analyzed by rapid PCR (polymerase chain reaction) assay, which provides results within 1.5-2 hours and has superior sensitivity (88-92%) compared to culture-based methods 1, 2.
Screening Methodology
Primary Screening Approach
Nasal swab with PCR analysis is the gold standard for MRSA screening because:
- Rapid turnaround time: Results available in 1.5-2 hours versus 20-72 hours for culture-based methods 2, 3
- High sensitivity: 91.7% sensitivity and 93.5% specificity when detecting MRSA directly from nasal specimens 2
- Superior performance: PCR screening within 6 days of clinical cultures shows 88% sensitivity versus only 65.5% for chromogenic medium 4
- Excellent negative predictive value: 97.1% NPV, making it highly reliable for ruling out MRSA colonization 2
Alternative Culture-Based Methods
If PCR is unavailable, culture-based screening options include (in order of performance):
- Oxacillin screen agar: 98.91% sensitivity - most reliable culture method 5
- MeReSA (MRSA-selective) agar: 97.82% sensitivity 5
- Mannitol salt agar with oxacillin: 89.13% sensitivity 5
- Broth-containing protocol (ASM method): Detects 19-32% more MRSA-positive specimens than direct plating but is 2-2.5 fold more expensive 6
Clinical Context for Screening
When to Screen
Screen patients before elective cardiac and orthopedic surgery according to local epidemiology 1. The decision between:
- Universal screening: All patients undergoing high-risk surgery
- Targeted screening: Based on clinical risk factors (past infection, colonization at other sites)
Should be determined by local MRSA prevalence, available resources, and organizational capacity 1.
Critical Timing Considerations
Screen within 6 days of anticipated need for results - sensitivity drops significantly from 79% to 46% when screening occurs 7+ days before clinical cultures are obtained 4. This timing is crucial for surgical planning and empiric antibiotic decisions.
Clinical Utility Beyond Screening
The nasal PCR swab has high negative predictive value for ruling out MRSA infection at various sites 7:
- Urinary tract infection: 100% NPV
- Bacteremia: 97.9% NPV
- Pneumonia: 97.8% NPV
- Wound infection: 92.1% NPV
- Overall infections: 98.5% NPV
This allows early de-escalation of empiric anti-MRSA antibiotics when the nasal screen is negative, supporting antimicrobial stewardship 7.
Important Caveats
- PCR vastly outperforms chromogenic medium - if your facility uses chromogenic culture methods, view results with caution as sensitivity is significantly lower 4
- Screening alone without decolonization does not reduce MRSA infections - one study showed universal rapid screening without subsequent decolonization failed to reduce nosocomial MRSA infections 1
- Effectiveness varies by endemicity: Rapid PCR testing reduces isolation requirements by >90% in low-endemicity settings but only 20% in high-endemicity settings 3
- Post-admission acquisition: Nearly 60% of infected patients may be MRSA-free on admission but acquire it during hospitalization, highlighting that screening addresses only part of transmission control 1