Contribution of rapid antimicrobial susceptibility testing from positive blood cultures to appropriate antibiotic treatment

Abstract

Introduction: This study aimed to evaluate the efficiency of antibiotic disks that are used to identify microorganisms, and compare rapid antimicrobial susceptibility testing (RAST) with routinely used automated systems in terms of time and accuracy.

Methodology: Antibiotic disk zone diameter results were determined at the 4^th, 6^th, 8^th, or 16^th–20^th hours in samples taken from blood culture bottles that were a positive for the presence of microorganisms, and conventional methods were used to provide the initial diagnosis at this stage.

Results: The laboratory received a total of 5,049 blood culture bottles within a period of 5.5 months. During this period, 2,170 blood culture bottles were positive for the presence of microorganisms. The maximum sample reading time for bacteria was 6–8 hours for Gram-negative bacteria, 16–20 hours for Streptococcus pneumoniae, and 8 hours for other Gram-positive bacteria. The time (hours) difference between RAST and the automated identification system was as follows: Escherichia coli (20.9 hours), Klebsiella pneumoniae (20.8 hours), Acinetobacter baumannii (29.7 hours), Pseudomonas aeruginosa (19.2 hours), Staphylococcus aureus (15.3 hours), Enterococcus faecalis (10.8 hours), Enterococcus faecium (14.5 hours), S. pneumoniae (37 hours). S. pneumoniae demonstrated the greatest time advantage among the isolates tested, yielding results up to 37 hours earlier with the RAST method.

Conclusions: Although all Gram-negative bacteria could be reported using the RAST method by the end of the 8^th hour, a significant proportion of Gram-positive bacteria still required waiting until the 16^th–20^th hour.