Direct antimicrobial susceptibility testing from positive blood culture bottles in laboratories lacking automated...

Timely initiation of appropriate antimicrobial can improve the outcome in terms of reduced morbidity and mortality in addition to reduced health-care costs. Availability of early preliminary Antimicrobial Susceptibility Test (AST) report will be useful in directing antimicrobial therapy. The aim of the study was to correlate AST by disc diffusion method, directly from positively flagged blood culture bottles, with the AST by automated method.

A total of 144 aerobic blood culture bottles flagged positive by the automated blood culture system were processed. The bacteria were pelleted by two-step centrifugation of the broth from the bottle and used to make a smear for Gram stain as well as an inoculum for antimicrobial sensitivity testing by Kirby Bauer disc diffusion method. Automated identification and AST were also carried out.

On direct staining, 94 samples showed gram-negative bacilli, 39 showed gram-positive cocci, and 11 showed yeasts or polymicrobial growth. In the case of gram-negative bacteria, there was 99% categorical agreement between direct sensitivity testing and automated sensitivity testing with 1% disagreement. Among the gram-positive cocci, there was 96% categorical agreement with 4% disagreement between the two methods.

High degree of agreement between the two methods is promising and applicable to situations where automated sensitivity testing is not available. Even if the systems are available, this method would prove useful as an adjunct to standard AST reporting. This sensitivity report can be generated earlier than the conventional AST, enabling choice of appropriate antimicrobial.