Poster Presentation Australian Society for Microbiology Annual Scientific Meeting 2018

Evaluation of Short Incubation Disc Antimicrobial Susceptibility Testing (#233)

Li Ching Eng 1 , Nerissa Jia Yi Leong , Yasmin Binte Ibrahim , Lily Siew Yong Ng 1 , Thean Yen Tan 1
  1. Changi General Hospital, Singapore, SINGAPORE

Introduction and Aims

Bloodstream infections are one of the leading causes of mortality and morbidity in hospitals. Early availability of bacterial identification and antibiotic susceptibility results would reduce mortality, morbidity and hospital costs. The aims of this study were to evaluate the accuracy of short incubation (<10 hours) antibiotic susceptibility testing (AST) for positive blood cultures as compared to full incubation (16 - 20 hours). The performance standard of Muller Hinton (MH) and Muller Hinton with Thiazolyl Blue Tetrazolium Bromide (MH+TB) was also compared.

Materials and Methods

A total of 249 Gram-negative bacilli consisting of Enterobacteriaceae (n=209), Acinetobacter baumannii (n=21) and Pseudomonas aeruginosa (n=19) were tested against 18 antibiotics. Mixture of ESBL (n=31), ampC (n=10) and CRE (n=12) positive strains were also tested. Following inoculation of agar plates using a 0.5 Mcfarland suspension, disc zone sizes were measured by an automated disc reader after 8 hours (Enterobacteriaceae), 10 hours  (Acinetobacter baumannii and Pseudomonas aeruginosa), and full incubation (16 -20 hours). For each time-point, the categorical susceptibility (S, I, R) for each antibiotic was interpreted using CLSI breakpoints.

Results and Discussions

MH showed an overall average concordance rate of 92.1% compared to MH+TB (90.9%) for 13,356 antibiotic-organism comparisons. The percentage of minor errors, major errors and very major errors for MH vs MH-TB were 6.20% vs 5.90%, 0.5% vs 1.3% and 0.60%  vs 0.85% respectively. Ampicillin-sulbactam, trimethoprim-sulfamethoxazole, cephalothin and cefuroxime have the lowest categorical concordance rates (<90%). This implied that the reporting of these 4 antibiotics for short incubation should be cautious.  On the other hand, amikacin and meropenem have the highest categorical concordance rates (>95%).


Short incubation AST proved to be reliable for testing Enterobacteriaceae, Acinetobacter baumannii and Pseudomonas aeruginosa.  MH demonstrated slightly better categorical concordance as compared to MH+TB. As MH is readily available commercially, it is useful for short incubation reporting. This study showed that the reporting of short incubation antibiotic susceptibility test results is highly feasible.