Rapid identification and antimicrobial susceptibility testing of Gram-positive cocci in blood cultures by direct inoculation into the BD Phoenix system

Abstract

Rapid identification and antimicrobial susceptibility testing (AST) of the causative agent(s) of bloodstream infections are essential for the selection of appropriate antimicrobial therapy. To speed up the identification and AST of the causative agent, the fluid from blood culture bottles of a Bactec 9240 instrument (Becton Dickinson) containing Gram-positive cocci was mixed with saponin. After a 15-min incubation, the bacteria were harvested and transferred to the appropriate panel of a BD Phoenix automated microbiology system (Becton Dickinson) for identification and AST. With this approach (referred to as the direct method), we concordantly/correctly identified 56 (82%) of 68 monomicrobial cultures using the results obtained with the method currently used in our laboratory (current method) as comparator. Two (3%) isolates could not be identified and ten (15%) were misidentified. Complete agreement, concerning clinical susceptibility categories and MIC values, between the AST results determined with the direct method and the current method was found for 32 (55%) of 58 isolates. The E-test indicated that the direct method yielded a correct susceptibility profile for 13 of the remaining 26 blood culture isolates. Therefore, a concordant/correct susceptibility profile (with all antimicrobial agents tested) was obtained for 45 (77%) of 58 cultures. The overall error rate amounted to 1.9%, with the majority (1.3%) of errors being minor. Importantly, the results obtained with the direct method were available 12-24h earlier than those obtained with the current method.

Timeliness and Accuracy of Reporting Preliminary Blood Culture Results: A College of American Pathologists Q-Probes Study of 65 Institutions

“…27,28 In this study participants did not report the extent of Gram smear reporting or criteria used for identifying discrepancies. Although newer molecularbased methods can rapidly and accurately identify a wide range of different microbial species directly from positive blood cultures, [12][13][14] the impact of this technology on management and outcomes of patients with bacteremia, beyond reporting preliminary Gram smear results, is yet to be determined. 29 Previous studies have shown excellent concordance between preliminary blood culture Gram stain interpretations and final culture results.…”

“…Continuously monitoring blood culture systems detect more than half of bacteremias in 24 hours or less, and the vast majority (85%) in less than 48 hours. 20 Although certain advances in direct identification of microorganisms from blood cultures have been described, 12,13,21 the current diagnostic mainstay in routine laboratory practice is performing Gram smears from broth bottles that have signaled positive growth. The Gram stain provides a preliminary result that is vital to effectively managing patients with serious infections and has more clinical importance than does final culture results.…”

“…8,11 Timely detection of bacteremia also facilitates initiation of rapid, direct methods for early identification and susceptibility testing. [12][13][14] Final culture and susceptibility reporting, which ordinarily occurs several days after onset of bloodstream infection, has been shown to have less impact on patient management and outcomes than preliminary Gram stain results. 15 Because rapid reporting of preliminary blood culture is linked to patient outcomes, it is generally considered standard laboratory practice to handle positive blood cultures as critical values that must be reported as soon as possible.…”

Direct disk diffusion test using European Clinical Antimicrobial Susceptibility Testing breakpoints provides reliable results compared with the standard method

“…Previously published direct AST protocols were validated using interpretative category breakpoints derived from Clinical and Laboratory Standards Institute (CLSI) and from DIN (Deutsches Institut für Normung), respectively [6,5,[10][11][12].…”

“…To overcome these disadvantages, molecular methods and mass spectrometry were evaluated for rapid identifi cation from positive fl agged blood cultures with sometimes confl icting results regarding the sensitivity and the lack of AST results [4]. Among the manufacturers of blood culture systems, only Becton-Dickinson (BD) provides a protocol for direct AST from positive blood culture bottles, which is time-consuming and useful only for Gram-negative rods [5,6].…”