Interpretive criteria for the agar diffusion susceptibility test with ofloxacin

Area of Science:

• Microbiology
• Clinical Pharmacology
• Antimicrobial Susceptibility Testing

Background:

• Accurate antimicrobial susceptibility testing is crucial for effective treatment of infectious diseases.
• Standardized methods are needed to correlate laboratory test results with clinical outcomes.
• Ofloxacin is a widely used fluoroquinolone antibiotic requiring reliable susceptibility testing.

Purpose of the Study:

• To determine the correlation between ofloxacin minimal inhibitory concentrations (MICs) and inhibition zone diameters.
• To establish reliable zone interpretation criteria for the NCCLS and DIN methods.
• To evaluate the accuracy of these interpretations in predicting bacterial susceptibility to ofloxacin.

Main Methods:

• Regression analysis of MICs and inhibition zones from 300 bacterial isolates across 15 species.
• Use of 5-microgram ofloxacin discs.
• Simultaneous testing using ICS/DIN and Kirby-Bauer/NCCLS methods on Mueller-Hinton agar.

Main Results:

• Correlation between MICs and zone sizes decreased with higher disc loads and with the Kirby-Bauer/NCCLS method.
• Recommended NCCLS zone interpretations: resistant ≤12mm, intermediate 13-15mm, susceptible ≥16mm.
• Recommended DIN zone interpretations: resistant ≤13mm, intermediate 14-17mm, susceptible ≥18mm.
• Minor errors in zone interpretation were low (0.7% NCCLS, 1.3% DIN).

Conclusions:

• The study provides validated zone interpretation guidelines for ofloxacin susceptibility testing using NCCLS and DIN methods.
• These guidelines aim to improve the accuracy of predicting bacterial response to ofloxacin therapy.
• The findings support the use of standardized zone interpretations for clinical decision-making.