Evidence for daptomycin Etest lot-related MIC elevations for Staphylococcus aureus

Area of Science:

• Clinical Microbiology
• Antimicrobial Susceptibility Testing

Background:

• Accurate antimicrobial susceptibility testing is crucial for guiding treatment decisions.
• Standardized methods like broth microdilution (BMD) are essential for reliable Minimum Inhibitory Concentration (MIC) determination.
• Etest is a widely used alternative method for MIC testing.

Purpose of the Study:

• To evaluate the agreement and identify potential biases between Etest and BMD for MIC testing of Staphylococcus aureus.
• To assess the performance of different Etest lots in comparison to BMD.

Main Methods:

• Simultaneous MIC testing of 587 Staphylococcus aureus isolates using Etest and BMD.
• Analysis of Etest to BMD MIC ratios to assess testing bias.
• Evaluation of categorical and essential agreement, as well as major and very major errors.

Main Results:

• A consistent bias towards higher Etest MIC values compared to BMD was observed (average ratio 1.3).
• Substantial variability in Etest performance was noted across 8 different Etest lots.
• While overall agreement was high, specific Etest lots contributed to most of the discrepancies, including very major and major errors.

Conclusions:

• Etest demonstrates a bias for higher MICs and exhibits lot-to-lot variability when compared to BMD for Staphylococcus aureus.
• Careful consideration of Etest lot performance is necessary to minimize potential errors in antimicrobial susceptibility testing.