Tick-tock, beat the clock: comparative analysis of disc diffusion testing with 6-, 10-, and 24-h growth for accelerated antimicrobial susceptibility testing and antimicrobial stewardship

Disc diffusion testing by Kirby-Bauer technique is the most used method for determining antimicrobial susceptibility in microbiological laboratories. The current guidelines by The Clinical and Laboratory Standards Institute (CLSI) 2022 specify using an 18- to 24-h growth for testing by disc diffusion. We aim to determine if using an early growth (6 h and 10 h) would produce comparable results, thus ultimately leading to reduced turnaround time. Six-hour, 10-h, and 24-h growths of 20 quality control strains and 6-h and 24-h growths of 48 clinical samples were used to perform disc diffusion testing using a panel of appropriate antimicrobial agents. Disc diffusion zone sizes were interpreted for all and comparative analyses were performed to determine categorical agreement, minor errors (mE), major errors (ME), and very major errors (VME) according to CLSI guidelines. On comparing with the standard 24 h of incubation, disc diffusion from 6-h and 10-h growths of quality control strains showed 94.38% categorical agreement, 5.10% mE, 0.69% MEs, and no VMEs. Disc diffusion testing for the additional 40 clinical samples yielded a similarly high level of categorical agreement (98.15%) and mE, ME, and VME of 1.29%, 1.22%, and 0% respectively. Disc diffusion testing using early growth is a simple and accurate method for susceptibility testing that can reduce turnaround time and may prove to be critical for timely patient management.