Diagnostic value of cefoxitin susceptibility test compared with other diagnostic methods of methicillin-resistant Staphylococcus aureus

Background: Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most important pathogen in the hospital and community. Therefore, a precise identification of the antibiotic-resistant strains is essential to control the infection and prevent the MRSA transmission rates. The aim of this study was to determine a diagnostic value of cefoxitin susceptibility test compared with the oxacillin susceptibility test and E-test (Epsilometer test) for detection of MRSA. Materials and Methods: This study was conducted on 296 S.aureus isolated from the nasal specimen patients of referred to emergency department of Kashsn Shahid-Beheshti hospital. Resistance to methicillin was determined by oxacillin (1µg), and cefoxitin (30µg) disk diffusion methods based on CLSI guideline and according to no growth zone size and the minimum inhibitory antibiotic concentration by E-test. PCR assay was used as a gold standard for detecting mecA gene in MRSA isolates. Results: Thirty-two (10.8%), 28 (9.5%), 30 (10.1%), 26 (8.8%) out of 296 S.aureus were considered as MRSA strains using the oxacillin and cefoxitin disk diffusion methods, E-test and PCR, respectively. Sensitivity, specificity, positive and negative predictive values for the oxacillin diffusion method were 100% , 97.7% , 81.2% and 100%; for the cefoxitin disk diffusion method 96.2%, 98.8%, 89.3% and 99.6% and for E-test 100%, 98.5%, 86.7% and 100%, respectively. Conclusion: PCR assay is the best method for detecting MRSA; however, it is an expensive method. Phenotypic methods, especially the cefoxitin disk diffusion method, can be a good alternative to PCR for detection of MRSA.