This is a Microsoft CoPilot assisted summary of a UNMC ID Infection Control/Antimicrobial Stewardship Journal Club presentation developed and presented by Dr. Jasmine Marcelin on 1/7/2026. Special thanks to Dr. Tess Karre for her input in preparing the presentation.
Timely and accurate antimicrobial susceptibility testing (AST) is critical for managing bloodstream infections, and delays in initiating appropriate therapy can lead to serious complications. Conventional phenotypic AST, while reliable, can take up to 72 hours to result, and this can mean the difference between recovery and deterioration.
A recent study by Nanishi et al. at The Hospital for Sick Children evaluated the performance of direct-from-blood culture phenotypic AST using the BD Phoenix™ M50 system. This approach bypasses the need for subculturing colonies, enabling susceptibility testing directly from positive blood culture broth.
Study Highlights
- Design: Retrospective review of 135 monomicrobial Gram-negative bacteremia cases (2021–2023).
- Method: Direct AST results compared to conventional AST using the same Phoenix panel.
- Metrics: Essential Agreement (EA: do MIC’s agree?), Categorical Agreement (CA: do interpretations agree?), and error rates (very major error [VME]= false susceptible, major error [ME] = false resistant, minor error [mE] = method discordance with intermediate).
Key Findings
- Turnaround Time: Direct AST reduced reporting time by ~24 hours compared to conventional methods (median 14 h vs. 38 h; P < 0.0001).
- Accuracy: EA and CA were 99.5% and 99.6%, respectively.
- Safety: No very major errors (false-susceptible); major and minor error rates were only 0.25%.
- Implication: Faster results without sacrificing accuracy: critical for optimizing therapy and supporting antimicrobial stewardship.
My Perspective
I’m not a pediatric infectious diseases specialist, but this topic caught my attention because rapid AST has the potential to transform care across all age groups. In my own practice, I’ve seen how delays in susceptibility results can lead to prolonged empiric therapy, unnecessary broad-spectrum antibiotic use, or complications from inadequate treatment. Currently, we use the MicroScan WalkAway system for AST at our institution, but there are studies with this system demonstrating similar results. This study demonstrates that with minimal workflow adjustments, may can achieve faster, accurate results using technology many labs already have. However, it is important to note that these methods are considered laboratory-developed tests (LDTs), which introduces regulatory complexity. There are already FDA approved tests like the Vitek Reveal system that would have less regulatory issues, and produce results in half the time of the modified approach that was used in this study. The issue is that those may require more upfront capital investment, but possibly the investment would be worth the downstream clinical benefits.
Limitations: The time reduction noted was focused on time to reporting in the lab system rather than the time to reporting in the EHR, therefore it is still unclear to what extent this process impacted time to appropriate antibiotic treatment and other important clinical outcomes. Additionally, there was a low baseline rate of antimicrobial resistance in general, which means that there was a lower likelihood of the test returning major errors (false resistant). Generalizability is limited based on these findings and the fact that the sample size was quite small, with only some of the eligible blood cultures were used due to micro lab staffing. The study did not spend time discussing antimicrobial stewardship practices as a result of this lab procedural change, which would be a very important component of translating to clinical outcomes, as many studies have shown that diagnostic testing changes without ASP intervention is not as effective as when ASP intervention is paired. This reinforces the importance of integrating rapid AST into antimicrobial stewardship programs, not just for speed, but for smarter, safer care.
What Does This Mean for Institutions?
The big question for institutions considering how to speed up their AST processes is whether to:
- Adapt a direct AST workflow using their existing conventional AST platform for faster results, or
- Invest in FDA-approved rapid platforms like Vitek Reveal, which deliver results in under 8 hours and reduce compliance burden.
While direct AST is cost-effective and leverages existing infrastructure, FDA-approved systems offer speed and simplicity with fewer regulatory hurdles. For most institutions, this decision will hinge on balancing clinical impact, cost, and compliance, and ensuring antimicrobial stewardship teams are ready to act on rapid results.
Bottom Line: Direct-from-blood culture AST using BD Phoenix M50 (and similar approaches with other conventional AST systems) can dramatically shorten wait time to results. But institutions must weigh speed, cost, regulatory requirements, and infrastructure when deciding between modified workflows and fully approved rapid platforms.