A number of studies have shown cefoxitin discs to be more sensitive and specific than oxacillin discs in confirming MRSA. It gives fewer misleading results with BORSA and fits easily into laboratory routine. Our Rosco products also benefit from extreme stability (up to 4 years when stored at ambient temperature) with uniform and reliable results.
Why is Cefoxitin more accurate than Oxacillin?
The distribution overlap between MRSA and MSSA MIC's that occurs with Oxacillin is absent with Cefoxitin
Cefoxitin detects all staphylococci which are mecA positive. BORSA strains are mecA negative but hyperproduce beta lactamase and consequently appear resistant to oxacillin, they are sensitive to cefoxitin so are easily screened out.
Cefoxitin is easier to use than Oxacillin as it does not require low temperature incubation or high salt content media
In use comparison
| Oxacillin | Cefoxitin | |
| Incubation temperature | 30°C | 35°C |
| Culture Medium | Specific media required | Standard sensitivity medium |
| Sensitivity/Specificity % | 99/90 | 100/99 |
Who recommends the use of Cefoxitin rather than Oxacillin for MRSA confirmation?
EUCAST (guidelines for the detection of resistance mechanisms) and the European Antimicrobial Resistance Surveillance Network (EARS-Net). SRGA and CLSI both adopted Cefoxitin for MRSA screening in 2004
EARS-Net have published a protocol for testing Staphylococcus aureus direct from positive blood cultures
For reliable detection of MRSA the protocol recommends the use of a cefoxitin disk diffusion test. The specification is Cefoxitin 10µg potency disk on ISA (Standardised sensitivity test agar) or Mueller Hinton agar with a semi-confluent growth.
Interpretation
| Sensitive | Resistant | |
| ISA | ≥ 22 mm | < 22 mm |
| MHA | ≥ 18 mm | < 18 mm |
Disk identification CFO10, potency 10µg.
Cefoxitin disks are available in both 10µg and 30µg potencies, Skov (2005) reports the 10µg disks performed slightly better with clearer zone edges for reading
References
Skov, R. et al. Evaluation of a cefoxitin 30 µg disc on Iso-Sensitest agar for detection of methicillin-resistant Staphylococcus aureus. Journal of Antimicrobial Chemotherapy (2003) 52: 204–207
Felten, A. et al. Evaluation of Three Techniques for Detection of Low-Level Methicillin-Resistant Staphylococcus aureus (MRSA): a Disk Diffusion Method with Cefoxitin and Moxalactam, the Vitek 2 System, and the MRSA-Screen Latex Agglutination Test. Journal of Clinical Microbiology (2002) 40(8): 2766–2771
Skov, R. et al. Evaluation of 10, 30 and 60 mcg cefoxitin Neo-sensitabs for prediction of methicillin resistance in S. aureus. ESCMID,15th European Congress of Clinical Microbiology and Infectious Diseases 2005, Vol. 11, Supp. 2. P1380
