Rapid identification and antimicrobial susceptibility testing directly from blood cultures of gram-negative and gram-positive isolates.

BACKGROUND

Bloodstream infection is still an important cause for morbidity and mortality. In order to reduce the turnaround time for laboratory diagnosis of bacteremia, the efficacy of identification and antimicrobial susceptibility testing using samples taken directly from positive culture bottles by Vitek 2 compact and Kirby-Bauer disk diffusion method was evaluated.

METHODS

A total of 218 monomicrobial blood cultures, including 81 Gram-negative and 137 Gram-positive isolates, were detected by a direct (that is, the organism was taken from the positive blood culture bottle) and standardized (taken from the overnight agar medium subculture) method on Vitek 2 compact ID cards for identification and Kirby-Bauer disk diffusion method for antimicrobial susceptibility testing.

RESULTS

79 of 81 Gram-negative rods (97.5%) and 84 of 137 Gram-positive cocci (61.3%) were correctly identified to the species level. Among 41 strains of wrongly identified Gram-positive cocci, 30 strains (73.2%) were misidentified as Kocuria. For antimicrobial susceptibility testing, the direct method had an overall error rate of 2.1% for Gram-negative rods, with 0.1% very major, 0.4% major, and 1.6% minor discrepancies compared with the standard method. The overall error rate for Gram-positive cocci was 4.4%, with 0.2% very major, 1.3% major, and 2.9% minor discrepancies.

CONCLUSIONS

Direct identification and antibiotic susceptibility testing with samples taken directly from blood cultures provided excellent results for Gram-negative rods and decreased turnaround time, while it would be less reliable for identification of Gram-positive cocci, although their antibiotic susceptibility testing yielded very good results.