Emergency Medicine: Open Access
Open Access

Abstract

The Discrepancy of the Minimum Inhibitory Concentration Results for Methicillin-Resistant Staphylococcus aureus by Various Measurement Methods

Yuka Kitano, Shigeki Fujitani, Haruaki Wakatake, Machi Yanai, Sari Umekawa, Yosuke Homma and Yasuhiko Taira

Background: It has been postulated that there exists a discrepancy of methicillin-resistant Staphylococcus aureus (MRSA) minimum inhibitory concentration (MIC) results from various measurement methods. The association between higher MRSA MIC (MIC ≥ 2 μg/ml) and worse clinical outcome has been previously reported. Therefore, it is clinically essential to investigate whether such a discrepancy exists between the different MIC measurement methods.

Methods: From November 2009 to March 2011, 55 MRSA isolates were prospectively obtained at two emergency departments in Japan. The MIC of the isolates were measured by Etest® and five broth microdilution (BMD) methods, namely Eiken®, MicroScan® prompt method, MicroScan® turbidity method, Phoenix® and Vitek2® system, respectively. The MIC results of vancomycin, teicoplanin, linezolid, daptomycin and quinupristin-dalfopristin (Q-D) were evaluated. Statistical analysis was performed using the Wilcoxon signed rank test and Bland-Altman’s analysis.

Results: There was a tendency of constant and significant discrepancy of MIC results of anti-MRSA antibiotics between Etest® and the BMD methods for MRSA isolates. The averages of the vancomycin MIC were 1.86 μg/ ml in Etest® and 0.74 μg/ml in the Phoenix® method (p<0.01), respectively. For teicoplanin, they were 1.86 μg/ml and 0.60 μg/ml (p<0.01), and for linezolid they were 2.55 μg/ml and 1.18 μg/ml (p<0.01) with respect to Etest® and the Phoenix® method, respectively. Among the BMD methods, however, the MicroScan® prompt method and the MicroScan® turbidity method had less discrepancy from Etest® for vancomycin MIC measurement.

Conclusion: The MIC measured by various BMD methods tended to show consistently lower results compared to those measured by Etest®. Among the BMD methods, however, the MicroScan® prompt method and the MicroScan® turbidity method had less discrepancy from Etest® for vancomycin MIC measurement.