275. Methicillin-Resistant Staphylococcus aureus (MRSA) Vancomycin Susceptibility: Relationship Between Minimum Inhibitory Concentrations (MIC) by Etest and Broth Microdilution with Population Analysis Profile; Better Correlation with Etest
Session: Poster Abstract Session: Antimicrobial Susceptibility Testing
Friday, October 21, 2011: 12:15 PM
Room: Poster Hall B1
  • Khatib IDSA-11-SAB-MRSA-Susceptibility-Poster.pdf (46.3 kB)
  • Background:  

    Vancomycin (V) MICs for MRSA are often reported to be higher by Etest than by the broth microdilution (BMD) method. Whether Etest overestimates the MIC or provides more precise measurement is unclear.


    We measured V-MIC of 484 MRSA blood isolates by E-test and BMD, determined population analysis/area under the curve (PAP/AUC) of tested isolate/Mu3 (a control hVISA strain), and correlated MIC values with PAP/AUC ratios.


    Etest MICs were equal to BMD MICs in 214 (44.2%) isolates, one increment higher in 118 (24.4%) and one increment lower in 152 (31.4%) instances. PAP/AUC ratios steadily increased with increasing MICs by both tests. MIC-stratified PAP/AUC ratio means rose significantly with each increase in MIC value (p<0.001) including the extra Etest MIC increments (0.46±0.16 [MIC=1.5] vs. 0.26±0.16 [MIC=1] and 0.64±0.20 [MIC=2]; p<0.001). Comparing isolates with equal and discrepant Etest/BMD MICs revealed significant differences. Assuming that BMD MIC is the gold standard, PAP/AUC ratios among isolates with BMD MIC=2 mg/L (n=342), were 0.80±0.3, 0.64±0.2 and 0.50±0.2 with higher, equal (p=0.002) and lower Etest MICs, respectively (p<0.001). Among isolates with BMD MIC=1 mg/L (n=113), PAP/AUC ratios were 0.22 ±0.1 and 0.41±0.1 with equal and higher Etest MICs, respectively (p<0.001); isolates with lower Etest MIC were too few to analyze (n=2). Linear regression analysis revealed that MIC-PAP/AUC ratio correlation was better with Etest (regression slope [b] =0.312; R2=0.359 [Etest]; b=0.186; R2=0.0.291 [BMD]).


    Etest MICs correlate better than BMD MICs with PAP/AUC ratios and represent a more precise assessment of the isolate susceptibility. The extra Etest MIC values depict a more detailed analysis of the isolate population. Higher Etest MIC indicates the presence of less-susceptible populations rather than an overestimation of the MIC. This may explain the reported better correlation of Etest MICs with patient outcome.

    Subject Category: A. Antimicrobial agents and Resistance

    Riad Khatib, MD1, Kathleen Riederer, BS, MT2, Stephen Shemes, BS1 and Susan Szpunar, PhD1, (1)St. John Hospital and Medical Center, Grosse Pointe Woods, MI, (2)Medicine, St. John Hospital and Medical Center, Grosse Pointe Woods, MI


    R. Khatib, None

    K. Riederer, None

    S. Shemes, None

    S. Szpunar, None

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