Evaluation and comparison of tests to detect methicillin resistant S. aureus

Phenotypic methods for detection of methicillin resistant Staphylococcus aureus (MRSA) have been compared with the gold standard which, as of now, is by the detection of mecA gene and femA gene by polymerase chain reaction (PCR). Discrepancies in detection have an adverse effect on patient management, thereby highlighting the importance of accuracy in detection. Our study aims to evaluate the efficacy of cefoxitin disk diffusion test to detect MRSA and compare it with other phenotypic and molecular methods.

The study was conducted from June 2006 to December 2007 and included 610 Staphylococcus aureus (S. aureus) isolates obtained from clinical samples. All isolates were tested for MRSA using oxacillin screen agar plates with 6 microg/ml of oxacillin, cefoxitin disk diffusion using 30 microg disk and MIC of oxacillin. Selected isolates (55) were tested for presence of mecA gene and Fem A gene by PCR.

Out of 610 isolates, MRSA was identified in 34.09% by cefoxitin disk diffusion, 34.9% by oxacillin screen agar, 34.4% by MIC and 37.3% by oxacillin disk diffusion. When selected isolates were tested with molecular methods, the cefoxitin disk diffusion and PCR tests were comparable.

Prevalence of MRSA (34.09%) is quite high as in other studies. The oxacillin disk diffusion test which was used routinely earlier is showing low specificity (56%). Among all phenotypic methods, cefoxitin disk diffusion and PCR alone have similar sensitivity and specificity.

Results of cefoxitin disk diffusion test are in concordance with the PCR for mecA gene. Thus, the test can be an alternative to PCR for detection of MRSA in resource constraint settings.