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Rapid Phenotypic AST Q and A
Expert Q&A on Rapid Phenotypic AST for Bloodstream Infections

Released: December 04, 2025

Jose Alexander
Jose Alexander, MD, D(ABMM), CIC, FCCM, SM/MB(ASCP)
Jasmine R. Marcelin
Jasmine R. Marcelin, MD, FACP, FIDSA
Michael P. Veve
Michael P. Veve, PharmD, MPH
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Key Takeaways
  • Rapid phenotypic AST is helpful to quickly determine antimicrobial susceptibility when resistance is caused by a mechanism that is not a gene detectable by rapid multiplex testing.
  • With proper validation, rapid phenotypic AST has the potential to guide treatment in outpatient and inpatient settings.

Here, a multidisciplinary panel of healthcare professionals (HCPs) answers audience questions from a symposium discussing the role of rapid phenotypic ASTs in bloodstream infections.

If your institution already has an antibiogram to help make empiric decisions about antibiotics, how does rapid phenotypic antimicrobial susceptibility testing (AST) help to augment that?

Jose Alexander, MD, D(ABMM), CIC, FCCM, SM/MB(ASCP): The antibiogram is a tool that we use for guiding empiric treatment. We evaluate each patient’s risk of multidrug resistance and use it to decide the best treatment approach. The advantage of rapid phenotypic AST is that it provides a susceptibility profile that allows us to optimize the initial treatment that was originally based on the antibiogram.

We still need antibiograms to drive initial treatment, but the advantage of rapid phenotypic AST is that optimization can be done more quickly.

Jasmine R. Marcelin, MD, FACP, FIDSA: So, it is meant to supplement, not replace, the antibiogram.

Michael P. Veve, PharmD, MPH: I would add that if your practice site does not usually see Gram‑negative resistance, I do not know if you are going to see much additional value outside of using stewardship antibiogram services, where you could try to facilitate an early switch without susceptibility results.

Jose Alexander, MD, D(ABMM), CIC, FCCM, SM/MB(ASCP): There is a lot of discussion about limiting the use of rapid phenotypic AST to specific hospital units like the ICU. But the benefit of rapid phenotypic AST is based on the actions that we take. For example, in patients with high-risk infections, if they are severely immunocompromised there is a fear of de‑escalation. So a component to consider is the patient’s clinical condition. Even if you have evidence that an organism is pan susceptible, is that going to break through the fear causing HCPs to keep patients on broad-spectrum regimens a little longer?

What is the benefit of rapid phenotypic AST in patients with E. coli with no beta-lactamase enzymes detected?

Jose Alexander, MD, D(ABMM), CIC, FCCM, SM/MB(ASCP): When antimicrobial resistance is caused by a gene that is not present in rapid genotypic testing panels, you need to have a full phenotypic profile. Rapid phenotypic AST is probably the best technology for this because you get the final AST faster and it is based on the sum of how all resistance mechanisms are reflected in the phenotypic profiles.

What are the possibilities of using rapid phenotypic AST in the outpatient setting to help primary care providers make better decisions about antibiotics for urinary tract infections (UTIs)?

Michael P. Veve, PharmD, MPH: There is a great opportunity to expand the use of rapid phenotypic AST in the ambulatory setting, including with UTIs. The challenge is validating tests on urine cultures. A platform where you can reliably use rapid phenotypic AST in patients with UTIs would be a good future direction.

In resource‑limited settings where the rate of antimicrobial resistance is higher, what are ways that HCPs can help address this with rapid phenotypic AST?

Michael P. Veve, PharmD, MPH: I think that is a clear challenge. Improving access to the testing in resource-limited settings is step 1, and showing a benefit within these setting is step 2. I think there may be a larger incremental value in implementing rapid phenotypic AST in these settings, but we need more data and more funding to support this work.

What are your insights regarding faster communication between physicians and the microbiology department to optimize antimicrobial therapy?

Michael P. Veve, PharmD, MPH: I think that thoughtful communication in the electronic health record, such as pairing culture and susceptibility results with treatment recommendations or “nudges”, is what is really needed. In my experience, HCPs may struggle with what to do with the results they receive. Antimicrobial stewardship personnel can help facilitate the delivery of results to HCPs, but leveraging the electronic health record or advanced decision-making models are needed to guide HCPs on how to interpret these results. 

Jose Alexander, MD, D(ABMM), CIC, FCCM, SM/MB(ASCP): This is a critical component for implementing rapid phenotypic AST. This technology without the appropriate communication workflow would just increase expenses without direct clinical impact.

Are there any data on rapid phenotypic AST performance in the case of mixed infections?

Michael P. Veve, PharmD, MPH: Most rapid phenotypic AST platforms have the best performance with monomicrobial cultures; use of phenotypic AST in polymicrobial cultures is not recommended in many product labels owing to inaccuracy of results.

Jose Alexander, MD, D(ABMM), CIC, FCCM, SM/MB(ASCP): These tests are not intended to be used on polymicrobial cultures.

Your Thoughts
What methods do you currently use to guide antimicrobial selection in your practice? Leave a comment to join the discussion!

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