A comparison of reduced and standard incubation time for antimicrobial susceptibility testing by the disk diffusion method
DOI:
https://doi.org/10.61529/idjp.v34i4.457Abstract
Background: Antimicrobial resistance is a major worldwide health problem, and early availability of antibiotic susceptibility testing results has become vital for the planning of treatment. Despite being commonly utilized, the disk diffusion method requires an 18 to 24 hours incubation, resulting in delays in clinical decision-making.
Material and Methods: This prospective study was performed in the Microbiology Department at Shaukat Khanum Memorial Cancer Hospital and Research Centre, Lahore, between October 2024 and March 2025. One hundred clinical isolates were tested against selected antibiotics using CLSI disk diffusion breakpoints. The zones of inhibition were measured at 6 hours, 10 hours, and 18–20 hours. Categorical agreement (CA) and error rates were calculated later.
Results: In total, 600 drug–organism combinations were analyzed. CA improved from 83.8% at 6 hours to 92.2% at 10 hours. Error rates declined markedly between 6 hours and 10 hours (mE: 7.0% → 3.2%; ME: 5.7% → 2.2%; VME: 3.8% → 1.1%). There was a consistent CA of >90% for meropenem, nitrofurantoin, vancomycin, and linezolid at early incubation periods; however, ciprofloxacin for Acinetobacter and Enterococcus species displayed a low accuracy initially, which improved at 10 hours.
Conclusion: An early measurement of zone of inhibition at 10 hours of incubation in disk diffusion testing has demonstrated notably reliable results for several therapeutically important drug-organism pairs. Particularly in resource-poor settings, AST reporting using shorter incubation times may enable timely initiation of targeted treatment and improvement in antimicrobial stewardship.
Keywords: Antibiotic susceptibility testing, Disk diffusion, Reduced incubation
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Copyright (c) 2025 Nida Safdar; Aqib Sultan, Nasrullah Malik, Summiya Nizamuddin
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