Rapid direct disk diffusion testing for antibiotic resistance in urinary tract infections: a bacterial concentration-adjusted approach

Abstract

Antibiotic susceptibility testing (AST) for urinary tract infections typically requires time-consuming standardization of bacterial suspensions, delaying targeted treatment. Prior direct susceptibility testing (DST) approaches have largely overlooked the inoculum effect, testing at a single, arbitrary concentration and risking misclassification of both resistant and susceptible isolates. This study presents two rapid disk diffusion methods that systemically incorporate bacterial concentration into the analysis, enabling direct testing from urine samples without prior inoculum adjustment. Both approaches demonstrated high agreement with standard AST and reduced diagnostic time by up to 24 hours. These concentration-aware methods may streamline susceptibility testing workflows, particularly in resource-limited settings, and represent a practical advancement toward faster, clinically reliable DST.