Rapid identifcation 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 Vitek® 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 Vitek® 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 92% with less than 3% very 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 Vitek® 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.