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Updated: Jan 20, 2026
A Broth Microdilution Method for Antibiotic Susceptibility Testing of Gram-Positive Bacteria
Published on: August 29, 2025
Comparison of disk diffusion, MIC test strip and broth microdilution for eravacycline susceptibility testing of 422
Xinying Wang1,2, Xunlu Yin3,4, Yanjun Liu1
1Department of Laboratory Medicine, The Affiliated Taian City Central Hospital of Qingdao University, Taian 271000, China.
Objective:
To evaluate the reliability of different antimicrobial-susceptibility testing methods for determining the susceptibility of Staphylococcus aureus to eravacycline.
Methods:
Using broth microdilution (BMD) as the reference method, disk diffusion (DD) and MIC test strips (MTS) were employed to test 422 S. aureus isolates (256 MSSA and 166 MRSA). Interpretive breakpoints for eravacycline followed the ChinaCAST criteria (MIC ≤ 0.25 mg/L or inhibition zone diameter ≥ 20 mm = susceptible). BMD results were used to calculate categorical agreement (CA), essential agreement (EA), major errors (ME), and very major errors (VME) for each comparator method.
Results:
The susceptibility rates to eravacycline were 93.6% (BMD), 96.9% (MTS) and 93.1% (DD). Compared with BMD, MTS yielded CA 96.7%, EA 63.0% and VME 3.3%, whereas DD showed CA 98.1%, ME 1.2% and VME 0.7%. Stratified analysis revealed that MRSA was the main source of VME (7.8% for MTS and 1.8% for DD). Disk diffusion showed excellent agreement with BMD (κ = 0.85, 95% CI 0.74-0.95), whereas MTS yielded good agreement (κ = 0.64, 95% CI 0.46-0.81).
Conclusion:
Under the conditions of this study, disk diffusion demonstrated excellent categorical agreement with BMD and can serve as an alternative method for routine laboratory detection of eravacycline susceptibility.
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