From the Guidelines
MRSA PCR is more sensitive than nasal culture for diagnosing MRSA colonization, with a sensitivity range of 85-95% compared to 65-85% for traditional culture methods. The use of PCR testing can provide results within hours, allowing for faster clinical decision-making and earlier implementation of infection control measures 1. This is particularly important in settings where rapid identification of MRSA carriers is crucial for preventing transmission. Some studies have shown that rapid molecular methods using PCR to detect MRSA can have a beneficial impact on control of MRSA in ICUs 1.
Key points to consider when choosing between MRSA PCR and nasal culture include:
- Sensitivity: PCR is generally more sensitive than nasal culture
- Speed: PCR provides results within hours, while culture methods take 24-72 hours
- Cost: PCR is more expensive than culture methods
- Limitations: PCR cannot distinguish between viable and non-viable organisms, potentially leading to false positives
It's also important to note that neither nasal PCR nor culture alone is sufficient for diagnosing active MRSA infection - they only detect colonization. For diagnosing actual MRSA infection, specimens from the infected site must be cultured. The higher sensitivity of PCR makes it particularly valuable in high-risk settings like pre-surgical screening or outbreak investigations where rapid identification of carriers is crucial for preventing transmission.
From the Research
Comparative Sensitivity of MRSA PCR and Nasal Culture
- The sensitivity of MRSA PCR versus nasal culture for diagnosing MRSA infection has been evaluated in several studies 2, 3, 4, 5, 6.
- A study published in 2014 found that the MRSA PCR assay demonstrated 88.0% sensitivity and 90.1% specificity for predicting MRSA pneumonia 2.
- Another study published in 2015 reported a high sensitivity (93.3%) and negative predictive value (95.2%) of nasal PCR for MRSA in the lower respiratory tract 4.
- A comparison of two molecular methods (BD GeneOhm PCR assay and Easy-Plex) with two selective MRSA agars (MRSA-ID and Oxoid MRSA) for nasal swabs found that the molecular methods had similar sensitivities (86% and 84%) and were more sensitive than the chromogenic agars (62% and 84%) 5.
- A recent study published in 2023 found that the MRSA nasal PCR screen had a high negative predictive value (NPV) for ruling out MRSA pneumonia in critically ill surgical patients, with an NPV of 0.92 (0.75-0.98) in the ACS-ICU and 0.89 (0.73-0.96) in the CVICU 6.
Factors Influencing Sensitivity
- The sensitivity of MRSA PCR and nasal culture can be influenced by several factors, including the type of MRSA strain, the presence of other microorganisms, and the quality of the nasal swab or culture specimen 5.
- The use of selective agars can increase the sensitivity of nasal culture, but may also increase the risk of false positives 5.
- The timing of the nasal swab or culture specimen collection can also impact the sensitivity of the test, with earlier collection potentially increasing the sensitivity 4.
Clinical Implications
- The high sensitivity and negative predictive value of MRSA PCR make it a useful tool for ruling out MRSA pneumonia in patients with suspected infection 2, 4, 6.
- The use of MRSA PCR can help guide antibiotic therapy and reduce the duration of empirical MRSA-targeted therapy 3, 6.
- However, the positive predictive value of MRSA PCR is generally lower, and a positive result should be interpreted in the context of clinical symptoms and other diagnostic tests 2, 4.