Evaluation of MRSASelect (™) chromogenic medium for the early detection of methicillin-resistant Staphylococcus aureus from blood cultures.
BACKGROUND
Staphylococcus aureus bacteremia is associated with considerable morbidity and mortality. In theory, reducing the turnaround time in reporting of methicillin-resistant S aureus (MRSA) among patients with bactermia could assist with the rapid optimization of antimicrobial therapy.


OBJECTIVE
To evaluate the sensitivity and specificity of MRSASelect (Bio-Rad Laboratories, USA), a chromogenic medium, in the early detection of MRSA from blood cultures growing Gram-positive cocci in clusters, and to confirm that routine use of this medium would, in fact, reduce turnaround time for MRSA identification.


METHODS
The present study was conducted at three microbiology laboratories in Manitoba. Between April 2010 and May 2011, positive blood cultures with Gram-positive cocci in clusters visualized on Gram stain were subcultured to both MRSASelect and routine media. MRSA isolates were identified using conventional microbiological methods from routine media and using growth with the typical colony morphology (pink colony) on MRSASelect medium.


RESULTS
A total of 490 blood cultures demonstrating Gram-positive cocci in clusters on Gram stain were evaluated. S aureus was recovered from 274 blood cultures, with 51 S aureus isolates (51 of 274 [18.6%]) identified as MRSA. MRSASelect medium had a sensitivity of 98%, a specificity of 100%, a positive predictive value of 100% and a negative predictive value of 99.8% for the recovery and identification of MRSA directly from positive blood culture bottles. In addition, use of MRSASelect medium was found to improve turnaround time in the detection of MRSA by almost 24 h relative to conventional methods.


CONCLUSIONS
These data support the utility of MRSASelect medium for the rapid identification of MRSA from positive blood cultures. Further clinical studies are warranted to determine whether the improvement in turnaround time will result in a measurable reduction in suboptimal antimicrobial therapy and/or improvement in patient outcome.
Microbiology, Diagnostic Services of Manitoba; ; Department of Medical Microbiology, Faculty of Medicine, University of Manitoba;
Kanchana
K
Manickam
Microbiology, Diagnostic Services of Manitoba; ; Department of Medical Microbiology, Faculty of Medicine, University of Manitoba; ; Department of Medicine, Health Sciences Centre, Winnipeg, Manitoba.
Andrew
A
Walkty
Microbiology, Diagnostic Services of Manitoba; ; Department of Medical Microbiology, Faculty of Medicine, University of Manitoba;
Philippe Rs
PR
Lagacé-Wiens
Microbiology, Diagnostic Services of Manitoba; ; Department of Medical Microbiology, Faculty of Medicine, University of Manitoba;
Heather
H
Adam
Microbiology, Diagnostic Services of Manitoba;
Barbara
B
Swan
Microbiology, Diagnostic Services of Manitoba;
Brenda
B
McAdam
Microbiology, Diagnostic Services of Manitoba;
Peter
P
Pieroni
Microbiology, Diagnostic Services of Manitoba; ; Department of Medical Microbiology, Faculty of Medicine, University of Manitoba;
Michelle
M
Alfa
Microbiology, Diagnostic Services of Manitoba; ; Department of Medical Microbiology, Faculty of Medicine, University of Manitoba;
James A
JA
Karlowsky
Blood cultures; Chromogenic agar; MRSA; Staphylococcus aureus