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Spurious oxacillin resistance in Staphylococcus aureus because of defective oxacillin disks
J M Boyce1, J R Lonks, A A Medeiros
1Department of Medicine, Miriam Hospital, Providence, Rhode Island.
Journal of Clinical Microbiology
|July 1, 1988
Abstract:
Clinical isolates of Staphylococcus aureus wee inappropriately categorized as intermediate or resistant to oxacillin on the basis of tests with two lots of oxacillin disks. The potency of one lot was tested and found to be below accepted limits. Routine quality control tests failed to detect the defective disks.
Insights
Defective oxacillin disks led to incorrect Staphylococcus aureus susceptibility testing. Routine quality control failed to identify faulty disks, impacting clinical isolate categorization.
Area of Science:
- Microbiology
- Clinical Diagnostics
- Antimicrobial Resistance
Background:
- Accurate antimicrobial susceptibility testing (AST) is crucial for guiding Staphylococcus aureus treatment.
- Oxacillin disk diffusion is a standard method for detecting methicillin resistance in S. aureus.
- In vitro diagnostic test performance relies on the quality of reagents, including antibiotic disks.
Purpose of the Study:
- To investigate the cause of inappropriate categorization of Staphylococcus aureus clinical isolates.
- To evaluate the performance of oxacillin disks used in routine susceptibility testing.
- To identify potential failures in quality control procedures for diagnostic reagents.
Main Methods:
- Analysis of clinical isolates categorized as intermediate or resistant to oxacillin.
- Potency testing of specific lots of oxacillin disks.
- Review of routine quality control procedures for disk diffusion assays.
Main Results:
- Two lots of oxacillin disks were implicated in the miscategorization of S. aureus isolates.
- One lot of oxacillin disks was found to have sub-potent antibiotic content.
- Standard quality control measures did not detect the issue with the defective oxacillin disks.
Conclusions:
- Sub-potent oxacillin disks can lead to erroneous clinical interpretations of S. aureus resistance.
- Current routine quality control may not be sufficient to detect all defective antibiotic disks.
- Enhanced vigilance and improved quality control are necessary for reliable AST.