Rapid antimicrobial susceptibility testing (rAST) aims to deliver actionable susceptibility results within hours rather than the 24–48+ hours required by conventional culture-based methods, shortening the time to effective and optimally targeted antibiotic therapy—a determinant of outcome in bloodstream infection and sepsis.
Rapid antimicrobial susceptibility testing (AST) aims to determine which antibiotics will work against a bacterial infection in hours instead of the 24–48+ hours needed by traditional culture-based methods.
Traditional AST requires growing bacteria overnight, then testing drug susceptibility — often 1–3 days total.
Two categories of rapid testing
Molecular testing detects resistance genes or their products .by PCR, isothermal amplification, or whole-genome sequencing.
It is fast and can be run directly on blood, but only detects known mechanisms, and absence of a resistance gene does not confirm susceptibility—so it tells you what not to use rather than what will work.
Predictive value is high for Gram-positive organisms but more limited for Gram-negatives given the diversity of resistance mechanisms.
Phenotypic rAST measures bacterial growth (or metabolic activity) in the presence of antibiotic and works irrespective of the resistance mechanism, allowing true susceptible/resistant categorization and MIC determination.
The tradeoff is that organisms need time to grow and express their response, and species identification is generally required first.
Rapid phenotypic platforms deliver a susceptible/resistant answer in roughly 2–8 hours from a positive blood culture; for urine, results can come in as little as 30 minutes by loading the sample directly into a microchip.
EUCAST RAST allows direct disk-diffusion testing from positive blood cultures with readings at 4, 6, or 8 hours (and 16–20 h) using pathogen-specific breakpoints, currently validated for E. coli, K. pneumoniae, S. enterica, P. aeruginosa, S. aureus, S. pneumoniae, E. faecalis, E. faecium, and A. baumannii.
The greatest time savings come from bypassing steps: eliminating subculture (rapid AST from positive blood cultures) or bypassing blood culture entirely (ultra-rapid AST directly from whole blood), which can theoretically save 40–60 hours.
The evidence for improved patient-level outcomes remains mixed, even as workflow and stewardship benefits are consistent:
The FAST randomized clinical trial in Gram-negative bacteremia showed rapid AST enabled substantially earlier optimization of therapy—faster escalation of undertreated patients and faster de-escalation of overtreated patients—though it did not significantly change the primary clinical outcome.
A prospective service evaluation found rapid AST shortened time to optimal antibiotic (50 vs 69.5 h) and, among patients on ineffective empirical therapy, time to effective antibiotic (39.5 vs 57 h), and led to earlier discontinuation of aminoglycoside combination therapy, but with no difference in 28-day mortality or length of stay.
The Cochrane review concluded that although rapid testing offers a theoretical benefit of reduced time to targeted therapy and improved stewardship, substantial uncertainty in patient outcomes persists.
Stewardship guidelines note that rapid molecular assays paired with active stewardship intervention have been associated with faster time to effective/optimal therapy and, in some studies, reductions in mortality, length of stay, and cost.
The FAST trial illustrates how rapid AST shifts patients into appropriate therapy earlier:
A key limitation across all rapid phenotypic methods is the need for prior species identification, which is straightforward for monomicrobial infections but challenging in polymicrobial samples (sepsis, wounds, catheter-associated UTIs).
Current consensus holds that rapid, molecular, and AI-supported approaches complement rather than replace classical phenotypic testing, and their clinical value depends on integration with antimicrobial and diagnostic stewardship.
Delayed appropriate therapy increases mortality, especially in sepsis, and speeding this up helps combat antimicrobial resistance by enabling targeted (not broad-spectrum) treatment sooner