254. Direct disk diffusion susceptibility testing versus chromogenic agar methods for the detection of methicillin resistant Staphylococcus aureus in signal positive blood cultures
Session: Poster Abstract Session: Diagnostic Microbiology; Antimicrobial Sensitivities
Thursday, October 3, 2013
Room: The Moscone Center: Poster Hall C
Background: Staphylococci frequently cause bloodstream infections (BSI).  Appropriate antibiotics may be delayed because conventional testing methods take 48-72h to characterize blood culture isolates.  We evaluated three non-molecular approaches to differentiate MRSA from MSSA within 24h from positive blood cultures (BCs).

Methods: Aliquots from positive BC bottles growing Gram-positive cocci in clusters were inoculated to BBL™ CHROMagar™ MRSA (BD), Spectra™ MRSA* (REMEL), Mueller Hinton agar (MHA) and sheep blood agar plates.  Cefoxitin disks were placed on the inoculated MHA plates and all cultures were incubated for 24 h.  Catalase and slide coagulase tests were performed on all isolates to presumptively identify S. aureus.  Chromogenic agar plates were examined for colonies consistent with MRSA as per manufacturers’ instructions.   Cefoxitin disk diffusion zones of inhibition were measured on MHA plates and isolates were categorized as MRSA or MSSA using CLSI criteria.  All presumptive isolates were then definitively characterized using a commercial automated method (Gold Standard, GS).  The sensitivity and specificity for each method evaluated was then calculated relative to the GS method and statistical testing was performed using Chi-Square analysis.

Results:   Gram-stain identified 250 BC bottles growing GPCCL of which 78 were ultimately identified as S. aureus.  GS categorized 46 as MRSA and 32 as MSSA.  Compared to the GS, sensitivities and specificities were as follows: BD 100% and 87.5%; REMEL 100% and 78.1%; and direct susceptibility 97.8% and 93.7%.   None of these methods exhibited statistically different performance compared to the GS or each other.

Conclusion: Both chromogenic media-based approaches and the direct susceptibility approach performed similarly for the differentiation of MRSA from MSSA in approximately 24h.  While statistically significant differences were not observed, there was a trend for better specificity of the direct susceptibility and BBL™ CHROMagar™ MRSA methods compared to the Spectra™ MRSA* method.   Direct susceptibility testing on GPCCL represents a reliable approach to the differentiation of MRSA from MSSA in ~24h from signal positive blood cultures.

Jay Hudgins, DO MS, Melvin Weinstein, MD and Thomas Kirn, MD PhD, Medicine and Pathology, Rutgers - Robert Wood Johnson Medical School, New Brunswick, NJ


J. Hudgins, None

M. Weinstein, None

T. Kirn, None

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