Comparison of disk diffusion, MIC test strip and broth microdilution for eravacycline susceptibility testing of 422 Staphylococcus aureus isolates using ChinaCAST breakpoints.

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.