Rapid Identification and Susceptibility Testing of Gram-Negative Bacilli from Blood Cultures Using the Vitek 2 System

ABSTRACT

Providing rapid results for blood culture isolates is a critical function of clinical microbiology laboratories. This study evaluated the accuracy and turnaround time for identification and susceptibility testing of Gram-negative bacilli inoculated directly from positive blood cultures into the Vitekr 2 system. Direct inoculation was compared to conventional methods; which included biochemical tests; commercial identification systems and disc diffusion susceptibility testing. Two hundred and ninety-one of 327 isolates (89) were correctly identified to at least genus level by the direct Vitekr method. Susceptibility test results were compared for 3;925 organism antibiotic combinations. The overall rate of categorical agreement of direct and conventional antimicrobial susceptibility testing was 92with less than 3very major and major errors combined. The mean turnaround time for identification and susceptibility testing was 7.5 hours (SD 3.0 hours) compared to a mean of 32.3 hours (SD 14.7 hours) for the conventional method. These results suggest that direct inoculation of the Vitekr 2 system from blood cultures provides accurate; reliable identification and antimicrobial susceptibility results for the majority of commonly occurring Gram-negative pathogens; while the significantly reduced turnaround time should benefit patients and permit earlier rationalisation of antibiotic therapy; with a reduction in the use of broad spectrum antibiotics. A suggested protocol for routine use is included