Comparison of methods for the interpretation of disk diffusion susceptibility tests for augmentin

Area of Science:

• Microbiology
• Clinical Diagnostics
• Pharmacology

Background:

• Disk diffusion susceptibility testing is crucial for guiding antibiotic therapy.
• Accurate interpretation of Augmentin (amoxicillin/clavulanate) susceptibility is vital for treating urinary tract infections (UTIs).
• Standardized methods are needed to reliably assess bacterial resistance patterns.

Purpose of the Study:

• To compare the performance of Stoke's criteria, ICS method, and a modified error minimization technique for Augmentin disk diffusion tests.
• To establish a reliable urinary breakpoint for Augmentin susceptibility using disk diffusion.
• To evaluate the accuracy of these methods in classifying bacterial susceptibility and resistance.

Main Methods:

• Disk diffusion susceptibility tests for Augmentin were conducted on 239 patient isolates.
• Tests were interpreted using Stoke's criteria, the ICS method, and a modified error minimization technique.
• Minimal inhibitory concentration (MIC) served as the reference, with ≤16 mg/L indicating likely therapeutic success in UTIs.

Main Results:

• Zone radii of inhibition showed a strong correlation with minimal inhibitory concentration (MIC).
• The error minimization technique successfully established a urinary breakpoint (Zs ≥ 6 mm radius), with only 1% false susceptibility.
• A significant false-resistant rate (14%) was observed, necessitating alternative testing conditions for accurate resistance detection.

Conclusions:

• The error minimization technique offers a viable method for establishing Augmentin susceptibility breakpoints in UTIs.
• Current disk diffusion methods may overestimate resistance, requiring adjustments for accurate false-resistant rate determination.
• An indeterminate zone category (4 < Z1 < 6 mm) is recommended for resistance rates exceeding 20%.