The simplified agar dilution method (SAD) for simple and accurate fosfomycin MIC testing of urinary Escherichia coli isolates.

BACKGROUND

Fosfomycin is crucial for treating multidrug-resistant urinary tract infections (UTIs), but its rational use depends on accurate MIC testing. The reference agar dilution (AD) method is labor-intensive and not routinely adopted.

OBJECTIVE

To develop a Simplified Agar Dilution (SAD) method using commercial plates for fosfomycin MIC testing of urinary Escherichia coli.

METHODS

We included 100 clinical E. coli isolates and 4 QC strains. SAD plates were prepared by applying fosfomycin and G-6-P solution onto commercial Mueller-Hinton agar, followed by drying. All isolates were tested in parallel by SAD and standard AD. Uniformity was assessed by comparing QC strain MICs on different plate areas (surface and flipped bottom). Essential agreement (EA), categorical agreement (CA), and error rates were calculated.

RESULTS

QC strain MICs by SAD were within acceptable ranges and identical across all areas, confirming uniform drug distribution. For the 100 clinical isolates, the SAD method demonstrated an EA of 100% and a CA of 99% compared to the AD method. Only one minor error (1%) was observed, with no major or very major errors.

CONCLUSION

The Simplified Agar Dilution (SAD) method developed in this study is accurate, reliable, and significantly streamlines the testing procedure. It provides clinical microbiology laboratories with an efficient and practical standardized alternative for fosfomycin susceptibility testing.