alexa Detection of intermediately vancomycin-susceptible and heterogeneous Staphylococcus aureus isolates: comparison of Etest and Agar screening methods.


Clinical Microbiology: Open Access

Author(s): Riederer K, Shemes S, Chase P, Musta A, Mar A,

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Abstract Detection of Staphylococcus aureus isolates with intermediate vancomycin susceptibility (VISA) and heteroresistance (hVISA) remains problematic. The population analysis profile/area under the curve (PAP/AUC) is the gold standard but is cumbersome. We compared the performance of two Etest screening methods (macromethod [MAC] and glycopeptide resistance detection [GRD]) plus brain heart infusion (BHI) agars supplemented with 3 (BHI-V3) or 4 (BHI-V4) mg/liter vancomycin in detecting hVISA and/or VISA phenotypes. Etest hVISA screenings were done in parallel for 485 saved methicillin-resistant S. aureus (MRSA) blood isolates according to the manufacturer's instructions. The PAP/AUC was measured for all isolates according to the modified method. PAP/AUC test isolate/Mu3 ratios of <0.9, 0.9 to 1.3, and >1.3 were considered positive for susceptible MRSA (S-MRSA), hVISA, and VISA, respectively. PAP/AUC revealed seven VISA and 33 hVISA phenotypes. MAC screening was positive for 30 (75.0\%) hVISA/VISA and 49 (11.0\%) S-MRSA isolates. GRD screening was positive for 28 (70.0\%) hVISA/VISA and 63 (14.2\%) S-MRSA isolates. Growth on BHI-V3 was noted in all hVISA/VISA and 24 (5.4\%) S-MRSA isolates. Growth on BHI-V4 was noted in all VISA and four (12.1\%) hVISA isolates. None of the S-MRSA isolates grew on BHI-V4 agar. The sensitivity, specificity, and positive (PPV) and negative (NPV) predictive values were 75.0\%, 89.0\%, 38.0\%, and 97.5\% for MAC; 70.0\%, 85.8\%, 30.8\%, and 97.0\% for GRD; 100\%, 94.6\%, 62.5\%, and 100\% for BHI-V3; and 100, 99.2\%, 63.6\%, and 100\% for BHI-V4 (for detecting VISA). These findings suggest that both Etest screening methods have excellent NPV, but positive results require confirmation. BHI-V3 and BHI-V4 agars provide more precise identification of hVISA and VISA, respectively; they may be reasonable alternatives to PAP/AUC.
This article was published in J Clin Microbiol and referenced in Clinical Microbiology: Open Access

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