247. Vancomycin MIC of musculoskeletal Staphylococcus aureus isolates by Etest, Vitek2 and Broth Microdilution
Session: Poster Abstract Session: Diagnostic Microbiology; Antimicrobial Sensitivities
Thursday, October 3, 2013
Room: The Moscone Center: Poster Hall C
  • VancoMIC IDSA Poster 6x3.pdf (1.0 MB)
  • Background:

    Vancomycin minimal inhibitory concentration (MIC) read by Etest has been reported to be one dilution higher than that read by broth microdilution (BM) of Staphylococcus aureus (SA) from blood specimens. 


    SA isolates from bone and musculoskeletal tissue were frozen by our microbiology lab from 1991 to 1995. A total of 49 samples were thawed and grown on blood agar plates.  MICs were measured using BM method, Etest on Mueller Hinton agar and Vitek2. For mean and median measurements, MIC≤0.5 by Vitek2 were called 0.5. Agreement between the tests was measured by kappa statistic.


    Of 49 individual SA isolates tested (39 Methicillin Resistant SA, 10 Methicillin Susceptible SA), 83.7% and 85.7% had MIC ≤0.5 by BM and Vitek2 respectively.  By Etest, 85.7% of specimens had MIC in the 1-1.5 range. There was poor agreement in general between the tests with kappa<0.5 for all 2x2 comparisons. By BM, MIC mean was 0.49, median of 0.5. By Vitek2, MIC mean was 0.61, median was 0.5, while by Etest, MIC mean was 1.32 and median was 1.5. Etest results for the majority of specimens were 0.5-1.0 dilution higher than Vitek2 and BM MICs. Distribution of BM and Etest MIC by Vitek2 category is shown in table 1.


    Musculoskeletal SA isolates Vancomycin MIC measured by Etest is 0.5 to 1 dilution higher than that reported by Vitek2 or BM in this historic frozen cohort. This is similar to findings previously reported on blood specimens.


    Table 1. Vancomycin MIC of Staphylococcus aureus (Mean, Median, Mode)




    0.5 (n = 42)

    .4167, .25, 1.0

    1.2619, 1.5, 1.5

    1 (n = 5)

    .90, .50, 2.0

    1.5, 1.5, 2.0

    2 (n = 2)

    1.0, 1.0, 1.0

    2.0, 2.0, 2.0

    Wael Haidar, MD, Infectious Disease, Kansas University Medical Center, Kansas City, KS, Ryan Slayer, Kansas University school of Pharmacy, Kansas City, KS, Hital Shah, Kansas University School of pharmacy, kansas city, KS, Michael Brimacombe, PHD, Kansas University Medical Center, Kansas City, KS, Rebecca Horvat, PhD, Pathology and Laboratory Medicine, University of Kansas, Kansas City, KS and Wissam El Atrouni, MD, Infectious Diseases, Kansas University Medical Center, Kansas City, KS


    W. Haidar, None

    R. Slayer, None

    H. Shah, None

    M. Brimacombe, None

    R. Horvat, None

    W. El Atrouni, None

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