Evaluation of oxacillin resistance screening agar and chromogenic MRSA agar media for the detection of methicillin resistance in Staphylococcus aureus clinical isolates

: Methicillin-resistant Staphylococcus aureus (MRSA) strains which are the most frequent causes of hospital acquired infections, are also currently encountered with increasing frequency in community acquired infections. Therefore rapid and accurate identification of MRSA strains is essential in both implementation of infection control measures and prevention of the nosocomial spread of this microorganism. The aim of this study was to determine the specifisity, sensitivity, positive and negative predictive values of two commercial media, one was Oxacillin Resistance Screening Agar Base (ORSAB; Oxoid, England) and the other was chromogenic MRSA agar (BBL CHROMagar MRSA; BD, Paris, France), for the identification of MRSA strains. A total of 175 clinical S. aureus isolates, of which 45 were MRSA, and 130 were methicillin-susceptible S. aureus (MSSA), whose susceptibility to methicillin were determined by disk diffusion method using oxacillin and cefoxitin disks in Mueller-Hinton agar medium, were included in the study. When oxacillin disk diffusion test was accepted as the reference method, the specificity, sensitivity, positive and negative predictive values of ORSAB were found as 97.7%, 40%, 36.5% and 98.1%, respectively; while these values were detected as 95.5%, 37.6%, 35.7% and 96.1% for CHROMagar MRSA, respectively. These results indicated that both media may be used in laboratories where work load is high and the number of personnel is inadequate especially in screening studies together or in addition to another medium (mannitol-salt agar). However, since these methods exhibit low specifity (high false positive results), positive results should be confirmed using other methods such as disk diffusion, E-test or microdilution susceptibility testing.