Abstract
We describe results from a multicenter pilot study evaluating the eQUANTTM system for rapid generation of a standardized inoculum (0.5 McFarland equivalent) directly from blood cultures positive for target gram-negative bloodstream pathogens for same-day antimicrobial susceptibility testing (AST) by disk diffusion, MicroScan and Vitek2 methods. Out of 167 blood cultures that met study criteria (monomicrobial with one of 9 target gram-negative species), colony counts from eQUANT suspensions of 164 (98.2 %) were within the acceptable range of 1-2e8 ± 0.6 log10 CFU/mL (2.51×107 to 7.96×108 CFU/mL). Average eQUANT testing time was 72 min and AST results were available an average of 22.3 h earlier compared to next day testing. Overall performance of direct versus next day AST was excellent, with categorical agreement of 97.5 %, 96.9 % and 96.7 %, respectively, by disk diffusion, MicroScan and Vitek methods. For clinical application this could facilitate antibiotic stewardship and provide targeted antibiotic therapy recommendations earlier.
Introduction
The current time to when antimicrobial susceptibility test (AST) results are available for blood cultures is up to three days from specimen collection [1]. This long turn-around time results in empirical antimicrobial use until AST results allow assessment of efficacy and the need to either narrow or expand antimicrobial therapy. Observational studies examining time to treatment and mortality risk of patients with sepsis and septic shock have shown significant associations between poor outcomes and delayed or inappropriate antimicrobial therapy. In a retrospective analysis of over 35,000 patients with sepsis and septic shock admitted to Kaiser Permanente hospitals in California, delay in first antibiotic administration was associated with increased in-hospital mortality, with a linear increase in the risk of mortality for each hour delay in antibiotic administration [2]. Similar results were reported in an analysis of 50,000 patients in New York State hospitals, where for each hour that antibiotic administration was delayed there was a 4 percent increase in risk adjusted in-hospital mortality [3]. A prospective cohort study of 2124 patients demonstrated that antibiotic selection was inappropriate in 32 percent of the cases [4]. Mortality was significantly higher in patients receiving inappropriate antibiotic therapy compared with those who had received appropriate antibiotics (34 versus 18 percent).
Accelerating the time to availability of AST results is key to improving patient outcomes and to limit further emergence of antimicrobial resistance. Conventional laboratory workflow typically has AST performed from a standardized bacterial inoculum prepared from a subculture of a positive blood culture (PBC) onto solid media. This inoculum cannot be prepared directly from using traditional methods as blood interferes with the optical reading of the organism density, resulting in delayed AST results. Efforts have been made to speed up the process of generating standardized bacterial suspensions directly from PBCs, using methods that separate red blood cells from the bacteria and requiring centrifugation steps [5]. Direct from PBC dilution methods have also been explored but rely on unstandardized inocula that have been shown to result in MIC shifts, particularly for β-lactam antibiotics [6].
The eQUANT™ system (Avails Medical, Inc., Menlo Park, CA) is an automated platform which uses potentiometric sensing of changes in oxidation-reduction potential (ORP) from pathogen metabolism to prepare an organism suspension equivalent to a 0.5 McFarland density standard directly from positive blood cultures as rapidly as under one hour, thus eliminating the need for overnight subcultures of the PBC and reducing the time to AST results by a day compared to conventional methods. The eQUANT System was recently cleared by the US Food and Drug Administration for direct AST of positive blood culture samples containing an identified target species of gram-negative bacteria by disk diffusion with a panel of up to 12 antimicrobial agents [7]. In this multicenter pilot study, we assessed the eQUANT system’s utility in accelerating time to AST results directly from PBCs with gram-negative organisms. The accuracy of AST using eQUANT on the day of positivity of PBCs compared to AST performed from subcultures the next day was assessed by disk diffusion and two automated systems, Vitek 2 and MicroScan.
