Use of Positive Blood Cultures for Direct Identification and Susceptibility Testing with the Vitek 2 System

Abstract

In order to further decrease the time lapse between initial inoculation of blood culture media and the reporting of results of identification and antimicrobial susceptibility tests for microorganisms causing bacteremia, we performed a prospective study in which specially processed fluid from positive blood culture bottles from Bactec 9240 (Becton Dickinson, Cockeysville, Md.) containing aerobic media were directly inoculated into Vitek 2 system cards (bio-Mérieux, France). Organism identification and susceptibility results were compared with those obtained from cards inoculated with a standardized bacterial suspension obtained following subculture to agar; 100 consecutive positive monomicrobic blood cultures, consisting of 50 gram-negative rods and 50 gram-positive cocci, were included in the study. For gram-negative organisms, 31 of the 50 (62%) showed complete agreement with the standard method for species identification, while none of the 50 gram-positive cocci were correctly identified by the direct method. For gram-negative rods, there were 50% categorical agreements between the direct and standard methods for all drugs tested. The very major error rate was 2.4%, and the major error rate was 0.6%. The overall error rate for gram-negatives was 6.6%. Complete agreement in clinical categories of all antimicrobial agents evaluated was obtained for 19 of 50 (38%) gram-positive cocci evaluated; the overall error rate was 8.4%, with 2.8% minor errors, 2.4% major errors, and 3.2% very major errors. These findings suggest that the Vitek 2 cards inoculated directly from positive Bactec 9240 bottles do not provide acceptable bacterial identification or susceptibility testing in comparison with corresponding cards tested by a standard method.Detection of bloodstream infections is one of the most important tasks performed by the clinical microbiology laboratory. Rapid identification results and antimicrobial susceptibility tests are essential for guiding clinicians in the selection of the most appropriate treatment for patients with bloodstream infections (5, 22). There are few well-designed studies that have examined the clinical impact of rapid bacterial identification and antimicrobial susceptibility testing, and there is even less information on blood cultures. The available information suggests that early knowledge of these data may be related to lower mortality rates and could affect patient therapy, leading to reductions in health care costs for the pharmacy, laboratory, and other general charges (1,11,20,22).Many remarkable improvements have been made in an attempt to reduce the time required to identify pathogens in blood, and over the last several decades, a variety of automated systems have been developed for detecting or identifying pathogenic microorganisms as well as for determining antimicrobial susceptibility. Continuously monitoring automated blood culture systems have emerged as the new standard in blood culture technology, enabling detection of a positive blood culture within 24 h of initial incubatio...

Clinical Evaluation of QMAC-dRAST for Direct and Rapid Antimicrobial Susceptibility Test with Gram-Positive Cocci from Positive Blood Culture Bottles

“…These results indicate that the antimicrobial agreement rate of QMAC-dRAST from PBC with Gram-positive cocci was comparable or superior to that of other studies [9,12,21].…”

“…These direct AST methods using inoculation from PBCs had quite acceptable results for Gram-negative rods, but not for Gram-positive cocci [8,9,11,12,18]. For higher accuracy of direct AST result for…”

“…For reducing TAT, various researchers have tried to directly process the positive blood culture (PBC) samples without subculture involving overnight culture [7]. AST methods utilizing direct inoculation from PBC had previously been reported with significant agreement rates compared to reference methods [8,9].…”

Evaluation of the Merlin MICRONAUT System for Rapid Direct Susceptibility Testing of Gram-Positive Cocci and Gram-Negative Bacilli from Positive Blood Cultures

“…In general, good agreement between direct and standard susceptibility testing results was observed when gram-negative bacilli were tested, including both members of the Enterobacteriaceae and Pseudomonas species (4-6, 12, 14, 18, 21, 24, 26, 28). For direct testing of gram-positive cocci from blood cultures, only limited data from small studies are available for the VITEK, the VITEK 2, the Sensititre, and the MicroScan systems (5,6,8,18,26,29). A significantly higher rate of disagreement between direct and standard testing results for gram-positive cocci than for gramnegative bacilli was found.…”

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

“…In conclusion, despite the inoculum size, which is not strictly defi ned in direct AST but may strongly infl uence the AST results [6,12,20], the direct disk diffusion testing of blood culture isolates represents a simple and costeffective method of preliminary antimicrobial susceptibility testing. Using EUCAST guidelines for interpretation, the test system provides reliable results within 24 h. In the age of increasing antibiotic resistance, the direct AST results may be helpful in preventing treatment failure especially in vancomycin-resistant enterococci, ESBL-producing Enterobacteriaceae, multidrug-resistant Pseudomonas spp., and MRSA.…”