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Journal of Clinical Microbiology, March 2009, p. 566-568, Vol. 47, No. 3
0095-1137/09/$08.00+0     doi:10.1128/JCM.01566-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Interpretation of MRSASelect Screening Agar at 24 Hours of Incubation {triangledown}

Julie Carson,1 Betty Lui,2 Linda Rosmus,2 Howard Rennick,2 and Jeff Fuller2*

Department of Medical Microbiology and Immunology, University of Alberta,1 Division of Medical Microbiology, Department of Laboratory Medicine and Pathology, University of Alberta Hospital, Edmonton, Alberta, Canada2

Received 12 August 2008/ Returned for modification 27 September 2008/ Accepted 13 January 2009

An incubation time of 24 h at 35 to 38°C is recommended for the optimal performance of MRSASelect (Bio-Rad) chromogenic screening agar. An additional 24 h is required to capture slow-growing methicillin-resistant Staphylococcus aureus (MRSA). However, the normal hours of operation for most laboratories cannot reliably accommodate 24-h interpretation intervals. Daily agar plate interpretations are more likely to occur around 18 h and 42 h of incubation, which may compromise the performance of the chromogenic agar and negatively impact patient infection control efforts. In order to validate the importance of stringent incubation times to plate performance, we evaluated MRSASelect medium at controlled intervals of 24 and 48 h of incubation, using clinical MRSA-screening swabs. A total of 1,071 MRSA-positive and 2,733 MRSA-negative cultures were selected for analysis. Compared to 48-h-incubation results, the sensitivity and specificity of MRSASelect at 24 h were 98.3% and 98.2%, respectively. Only 19 of 1,071 (1.8%) MRSA-positive isolates required 48 h for detection. Holding 24-h negative plates an additional 24 h resulted in the workup of 253 (6.6%) pink, yet non-MRSA, colonies. The 24-h positive and negative predictive values of MRSASelect, assuming MRSA prevalences of 1, 5, and 10%, were 35.5 and 99.98%, 74.2 and 99.9%, and 85.9 and 99.8%, respectively. MRSASelect medium held for 24-h incubation is a highly sensitive and specific MRSA-screening tool. Further incubation prolongs the turnaround time for results and creates a significant amount of unnecessary work in the laboratory.

* Corresponding author. Mailing address: Division of Medical Microbiology, Department of Laboratory Medicine and Pathology, University of Alberta Hospital Walter C. Mackenzie Centre 2B3.13, 8440 112 Street, Edmonton, Alberta T6G 2J2, Canada. Phone: (780) 407-2767. Fax: (780) 407-3864. E-mail: jeff.fuller{at}capitalhealth.ca

{triangledown} Published ahead of print on 14 January 2009.

Journal of Clinical Microbiology, March 2009, p. 566-568, Vol. 47, No. 3
0095-1137/09/$08.00+0     doi:10.1128/JCM.01566-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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