Nasal Swabs Are the Primary Method for MRSA Testing
For MRSA screening, nasal swabs are the most commonly used sampling method as they are easy to use, non-invasive, and generally well tolerated without the need for local anesthesia. 1
Optimal Swab Types and Techniques
- Flocked swabs or cellular foam tip swabs perform significantly better than conventional rayon swabs for MRSA detection, with higher bacterial recovery rates 2
- The best results are obtained using flocked swabs in combination with Amies preservation medium 2
- Swab choice has a significant impact on laboratory results, potentially contributing to true positive or false negative detection of nasal MRSA carriage 2
- For proper MRSA screening, vigorous swabbing of the nares is required 1
Sampling Sites and Sensitivity
- Nasal swabs alone have limited sensitivity for MRSA detection (48% by culture and 62% by rapid PCR) 3
- Adding groin swabs increases sensitivity to 79% (culture) and 92% (PCR) 3
- Further adding throat swabs increases sensitivity to 96% (culture) and 99% (PCR) 3
- For comprehensive screening, multiple sites should be sampled including nares, any open skin lesions, groin, and throat 1, 3
Testing Methods
- Traditional culture-based methods require at least 20-24 hours until results are available 4
- Rapid PCR-based methods (like IDI-MRSA) can provide results in approximately 1.5 hours 4
- PCR-based methods have shown good diagnostic values: 91.7% sensitivity, 93.5% specificity, 82.5% positive predictive value, and 97.1% negative predictive value compared to culture-based methods 4
- For skin and soft tissue infections, positive nasal swab cultures for MRSA have 100% specificity and positive predictive value, but only 55% sensitivity 5
Transport Considerations
- Swabs can be transported in liquid Stuart's medium or Amies medium without charcoal 6
- When using Amies medium, an additional heating step may be required to liquefy the gel for proper processing 6
- Pooled swabs (from nose, axilla, and groin) transported in Amies medium have shown 85% sensitivity and 95% specificity compared to culture methods 6
Practical Application
- For screening of asymptomatic MRSA carriers, nasal swabs should be the primary method but should be supplemented with additional sites (groin and throat) for comprehensive detection 3
- For patients with skin and soft tissue infections, positive nasal swabs can help guide antimicrobial therapy when wound cultures cannot be obtained 5
- In outbreak situations, attempts at eradication of MRSA may include topical nasal mupirocin therapy (twice daily for 5-7 days) among colonized individuals 1
Pitfalls and Caveats
- Nasal swabbing alone misses a significant portion of MRSA carriers and should not be relied upon as the sole screening method 3
- The choice of swab type can significantly affect detection rates - using suboptimal swabs may lead to false negative results 2
- High-level resistance to mupirocin has been associated with subsequent failure of decolonization efforts 1
- Undirected nasal/nasopharyngeal swabs show poor correlation with properly directed swabs due to contamination from the nasal vestibule and cavity 1