Discordance of vancomycin minimum inhibitory concentration for methicillin-resistant Staphylococcus aureus at 2 μ g/mL between Vitek II, E-test, and Broth Microdilution

Chien-Feng Kuo; Chon Fu Lio; Hsiang-Ting Chen; Yu-Ting Tina Wang; Kevin Sheng-Kai Ma; Yi Ting Chou; Fu-Chieh Chang; Shin-Yi Tsai

doi:10.7717/peerj.8963

Discordance of vancomycin minimum inhibitory concentration for methicillin-resistant Staphylococcus aureus at 2 μ g/mL between Vitek II, E-test, and Broth Microdilution

PeerJ. 2020 May 11:8:e8963. doi: 10.7717/peerj.8963. eCollection 2020.

Authors

Chien-Feng Kuo^#¹, Chon Fu Lio^#², Hsiang-Ting Chen^{1

3}, Yu-Ting Tina Wang², Kevin Sheng-Kai Ma^{2

4}, Yi Ting Chou², Fu-Chieh Chang⁵, Shin-Yi Tsai^{2

3

6

7}

Affiliations

¹ Division of Infectious Disease, Mackay Memorial Hospital, Taipei, Taiwan.
² Department of Laboratory Medicine, Mackay Memorial Hospital, Taipei, Taiwan.
³ Department of Medicine, Mackay Medical College, Taipei, Taiwan.
⁴ Department of Pathology, School of Dental Medicine, University of Pennsylvania, Philadelphia, USA.
⁵ Infection control center, Mackay Memorial Hospital, Taipei, Taiwan.
⁶ Department of Health Policy and Management, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, Maryland, U.S.
⁷ Graduate Institute of Long-Term Care, Graduate Institute of Biomedical Sciences, Mackay Medical College, New Taipei City, Taiwan.

^# Contributed equally.

Abstract

Background: Vancomycin, the first line antibiotic for methicillin-resistant Staphylococcus aureus (MRSA) bacteremia, is often administered inappropriately when MIC is greater than 2 µg/mL, including 'susceptible' strains. This study assessed the discordance of vancomycin minimum inhibitory concentration (MIC) for methicillin-resistant Staphylococcus aureus (MRSA).

Methods: In total, 229 MRSA isolates from blood cultures collected between 2009 and 2015 at a tertiary hospital in Taiwan were examined. The MICs of vancomycin were measured using Vitek 2, E-test, and standard broth microdilution at the level of 2 µg/mL.

Results: The geometric mean of the MICs of hospital-acquired MRSA was higher than that of community-acquired MRSA (P < 0.001), with the exact agreement rates (with broth microdilution) at 2 µg/mL being 53.6% in Vitek 2 and 86.7% in E-test. Overall, E-test (98.1%) had more categorical accordance than did Vitek 2 (94.0%; P = 0.026). Vitek 2 had a tendency to overestimate MRSA in high-MIC isolates, whereas E-test inclined underestimation in low-MIC isolates. Surprisingly, the discordance rates of MRSA vancomycin MICs were higher in hospital-acquired isolates (13.3%-17.0%) than in community-acquired isolates (6.2%-7.0%).

Conclusion: The Infectious Diseases Society of America recommends the use of alternative antimicrobial agents when vancomycin MIC is ≥ 2 µg/mL; in this study, only 53.6% of the isolates tested using Vitek 2 showed a high MIC in the broth microdilution method. Accurate identification of the resistance profile is a key component of antimicrobial stewardship programs. Therefore, to reduce inappropriate antibiotic use and mitigate the emergence of resistant strains, we recommend using complementary tests such as E-test or Broth microdilution to verify the MIC before administering second-line antibiotics.

Strengths: (1) We compared the categorical agreement between different methods measuring MRSA MICs level. (2) Physicians should incorporate this information and consider a complementary test to verify the appropriateness of the decision of shifting vancomycin to second-line antibiotic treatment to improve patients' prognosis. (3) MRSA-vancomycin MICs at a cutoff of 2 µg/mL obtained using Vitek II exhibited a higher sensitivity level and negative predictive value than those obtained using E-test in the prediction of categorical agreement with standard broth microdilution.

Limitation: (1) Our research was based on a single hospital-based study. (2) The MRSA strains in this study were stored for more than 12 months after isolation. (3) We did not collect information on clinical prognosis.

Keywords: Antimicrobial stewardship; Broth Microdilution; Clinical and Laboratory Standards Institute (CLSI); Community-acquired MRSA; E-test; Hospital-acquired MRSA; Methicillin-resistant Staphylococcus aureus (MRSA); Microbial automatic identification system (Vitek II); Minimal inhibitory concentration (MIC ); The inter-test agreements.

Grants and funding

This study was supported by the Department of Medical Research at Mackay Memorial Hospital (funding numbers: MMH-105-80, MMH-107-146). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.