Accuracy of 0.25 µg/mL-Interval MIC Measurement of Vancomycin Among Staphylococcus aureus: a Pilot Study

We aimed to examine the accuracy of 0.25 μg/mL-interval minimum inhibitory concentration (MIC) measurement of vancomycin (VCM) for Staphylococcus aureus.

We collected microbiological data on S. aureus from June 2020 to November 2023 at Okayama Univer-sity Hospital, Japan. The VCM MIC values were determined by the microdilution method using Dry Plate Eiken (Eiken Chemical Co., Ltd, Tokyo, Japan) at ≤ 0.5, 0.75, 1, 1.25, 1.5, 1.75, and 2 µg/mL. We performed day-to-day reproducibility testing for S. aureus standard strain (ATCC 29213) and evaluated the within-run reproducibility and intertechnician errors among clinical isolates. Finally, we investigated the possibility of VCM creeping by reviewing the clinical data.

Fifteen day-to-day reproducibility tests revealed within one-tube differences. Within-run reproducibility testing was performed for 33 clinical isolates, and 32 isolates (97.0%) demonstrated within one-tube differences; complete agreement and one-tube differences were observed at 45.5% and 51.5%, respectively. Inter-technician reproducibility was assessed for 10 clinical isolates, and complete agreement and one-tube differences were ob-served at 20% and 80%, respectively. Of 19 cases receiving VCM treatment, one case showed an MIC elevation (≤ 0.5 to 1.25 µg/mL), suggesting VCM creeping.

We confirmed the accuracy of VCM MIC determination at 0.25 µg/mL increments among clinical isolates of S. aureus, based on the broth microdilution method.