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Published on: September 9, 2015
Accuracy of commercial and reference susceptibility testing methods for detecting vancomycin-intermediate
Jana M Swenson1, Karen F Anderson, David R Lonsway
1Clinical and Environmental Microbiology Branch, Division of Healthcare Quality Promotion, Centers for Disease Control and Prevention, Atlanta, Georgia 30333, USA. jswenson@cdc.gov
Journal of Clinical Microbiology
|May 8, 2009
Summary
Evaluating vancomycin-intermediate Staphylococcus aureus (VISA) detection, this study found most commercial MIC test systems and reference methods struggled to accurately differentiate VISA strains. Essential agreement varied, with some systems under- or overcalling resistance, impacting clinical decisions.
Area of Science:
- Clinical microbiology
- Antimicrobial resistance
- Infectious diseases
Background:
- Accurate detection of vancomycin-intermediate Staphylococcus aureus (VISA) is crucial for effective treatment of serious S. aureus infections.
- Several commercial MIC test systems and reference methods exist, but their performance in identifying VISA can vary.
- The Clinical and Laboratory Standards Institute (CLSI) broth microdilution (BMD) method is a standard reference for antimicrobial susceptibility testing.
Purpose of the Study:
- To compare the performance of six commercial MIC test systems and three reference methods against the CLSI BMD reference method for detecting VISA.
- To assess the accuracy and agreement of different methods in categorizing S. aureus isolates with varying vancomycin MICs.
Main Methods:
- A total of 129 S. aureus isolates with known vancomycin MICs (1-8 microg/ml) were tested.
- Six commercial MIC systems (Etest, MicroScan, Phoenix, Sensititre, Vitek Legacy, Vitek 2) and three reference methods (agar dilution, disk diffusion, vancomycin agar screen) were evaluated.
- Results were compared to the CLSI BMD reference method using Difco Mueller-Hinton broth.
Main Results:
- Essential agreement (+/-1 dilution) was high (90.6-100%) for most methods, except Vitek Legacy (90.6%).
- Commercial systems showed discrepancies: Sensititre, Vitek Legacy, and Vitek 2 tended to undercall resistance, while MicroScan, Phoenix, and Etest tended to overcall resistance.
- Disk diffusion misclassified all VISA strains as susceptible. The vancomycin agar screen showed limited utility for intermediate strains.
- Differentiating isolates with vancomycin MICs of 2 or 4 microg/ml proved challenging for most methods.
Conclusions:
- No single commercial MIC system or reference method consistently and accurately detected all VISA strains.
- Performance varied significantly, with several systems misclassifying resistance levels.
- The accurate differentiation of VISA remains a challenge, necessitating careful interpretation of results and potentially further method validation.
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