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Evaluation of Chromogenic MRSA medium, MRSASelect and Oxacillin Resistance Screening Agar for the detection of methicillin-resistant Staphylococcus aureus

Department of Medical Microbiology, Royal Infirmary of Edinburgh, 51 Little France Crescent, Old Dalkeith Road, Edinburgh EH16 4SA, UK

Correspondence to:
Banavathi V Subba Krishna, Department of Medical Microbiology, Royal Infirmary of Edinburgh, 51 Little France Crescent, Old Dalkeith Road, Edinburgh EH16 4SA, UK; bvsk{at}yahoo.com

Aims: An evaluation of Chromogenic MRSA medium (CMRSA), MRSASelect (MRSAS) and Oxacillin Resistance Screening Agar (ORSA) was performed to determine the optimum medium providing a rapid and sensitive method for methicillin-resistant Staphylococcus aureus (MRSA) detection.

Methods: A total of 632 clinical specimens were cultured on the three media in first phase of the study, while 720 clinical specimens were cultured on CMRSA and ORSA in the second phase.

Results: The sensitivity and specificity, respectively, of the media in the first phase were: CMRSA 88.9% and 98.45%; MRSAS 92.1% and 99.1%; ORSA (24 h incubation) 68.3% and 98.8%; and ORSA (48 h incubation) 85.7% and 96.3%. In the second phase the sensitivity and specificity, respectively, were CMRSA 91.2% and 98.6%; ORSA (24 h incubation) 58.9% and 98.2%; and ORSA (48 h incubation) 85.6% and 95.6%. The positive predictive values of the two chromogenic media were higher than that for ORSA. There were fewer false-positive results with the chromogenic media (1.4% for CMRSA and 0.8% for MRSAS) compared with ORSA (3.3%).

Conclusions: Performing latex agglutination tests on growth from chromogenic media provides results for 93.8% of MRSA isolates within 24 h. There is a small increase in cost of chromogenic media compared with ORSA (£28 for MRSAS, and £36 for CMRSA, per 1000 specimens) and direct agglutination tests (£80 per 1000 specimens). However early availability of MRSA screening results can reduce the burden of MRSA in hospitals because of early implementation of infection control measures.