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Rapid Identification and Antimicrobial Susceptibility Testing Directly from Blood Cultures of Gram-negative and Gram-positive Isolates by Haishen Kong, Shujin Zhang, Xiao Chen, Weili Zhang, Qing Yang, Yajie Fu, Yiyin Wang, Xuefen Li

Background: Bloodstream infection is still an important cause for morbidity and mor-tality. In order to reduce the turnaround time for laboratory diagnosis of bacter-emia, 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 identifi-cation 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% mi-nor 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 dis-crepancies.
Conclusions: Direct identification and antibiotic susceptibility testing with sam-ples taken directly from blood cultures provided excellent results for Gram-neg-ative rods and decreased turnaround time, while it would be less reliable for iden-tification of Gram-positive cocci, although their antibiotic susceptibility testing yielded very good results.

DOI: 10.7754/Clin.Lab.2013.121132