Tentative interpretive criteria for the diffusion susceptibility test using 30-microgram netilmicin disks

Area of Science:

• Medical Microbiology
• Antimicrobial Susceptibility Testing

Background:

• Accurate antimicrobial susceptibility testing (AST) is crucial for guiding effective treatment decisions.
• Netilmicin is an aminoglycoside antibiotic used to treat serious bacterial infections.

Purpose of the Study:

• To correlate disk diffusion zone diameters with minimum inhibitory concentrations (MICs) for netilmicin.
• To establish optimal breakpoints for the 30-microgram netilmicin disk diffusion test.

Main Methods:

• Disk diffusion assay using 10-microgram and 30-microgram netilmicin disks.
• Testing against 471 clinical bacterial isolates in cation-supplemented Mueller-Hinton broth.
• Regression line and error-rate bounded analysis for breakpoint determination.

Main Results:

• The 30-microgram netilmicin disk demonstrated better correlation with MICs compared to the 10-microgram disk.
• Recommended breakpoints: >= 17 mm for susceptibility (<= 8 µg/ml) and <= 13 mm for resistance (> 32 µg/ml).
• Potential for minor interpretive errors with Pseudomonas strains having intermediate MICs (16 µg/ml).

Conclusions:

• The 30-microgram netilmicin disk diffusion test is a reliable method for clinical AST.
• Established breakpoints provide a basis for accurate interpretation of netilmicin susceptibility.
• Awareness of potential errors in intermediate ranges, particularly for Pseudomonas, is important.