FAST — Rapid Antimicrobial Susceptibility Testing for Gram-Negative Bacteremia

1. Publication

• Title: Fast Antimicrobial Susceptibility Testing for Gram-Negative Bacteremia: The FAST Randomized Clinical Trial
• Acronym: FAST
• Year & Journal: JAMA, published online April 18, 2026
• Citation: Banerjee R, Komarow L, Li Y, et al; Antibacterial Resistance Leadership Group. Fast Antimicrobial Susceptibility Testing for Gram-Negative Bacteremia: The FAST Randomized Clinical Trial. JAMA. 2026. doi:10.1001/jama.2026.5487

2. Context & Rationale

Background: Gram-negative bacteremia is common, time-critical, and increasingly shaped by cephalosporin/carbapenem resistance; delays in definitive susceptibility data can delay both escalation to active therapy and de-escalation of unnecessary broad-spectrum treatment. Earlier prospective studies of rapid susceptibility testing (e.g., RAPIDS-GN) generally shortened time to optimal therapy without clearly reducing mortality or length of stay, but were largely conducted in lower-resistance, higher-resource settings.

Research Question/Hypothesis: Among hospitalized patients with gram-negative bacilli bloodstream infections, does rapid blood culture antimicrobial susceptibility testing (AST) improve clinical outcomes (Desirability of Outcome Ranking, DOOR) compared with standard testing, particularly in regions with high antimicrobial resistance?

Why This Matters: Tests whether a diagnostic speed advantage (rapid AST identifies resistance/susceptibility faster) actually translates into better patient-centered outcomes when combined with antimicrobial stewardship — not just faster antibiotic changes, but genuinely better DOOR.

3. Design & Methods

• Study Type: Multinational, open-label, randomized clinical trial
• Setting & Centers: Multiple sites including US and Spain (e.g., A Coruña), among others; multinational, multicenter design specifically incorporating high-antimicrobial-resistance regions
• Population:
• Inclusion: Hospitalized adults and children (all age groups; median age 72y) with bloodstream infections caused by gram-negative bacteria, detected via MALDI-ToF mass spectrometry
• Exclusions: Not detailed in available trial text; ~20% of patients had organisms not covered by the rapid AST panel
• Intervention: Rapid AST of positive blood cultures using a commercial platform (Vitek Reveal, bioMérieux) plus standard susceptibility testing (n=413)
• Comparator: Standard susceptibility testing alone (n=437)
• Randomization: 850 total patients; antimicrobial stewardship teams reviewed and recommended treatment in both groups (isolating the specific value of rapid diagnostics, not stewardship itself)
• Blinding: Open-label
• Statistical Power & Follow-Up: Primary outcome: Desirability of Outcome Ranking (DOOR) — an ordinal, patient-centered 30-day composite with 3 ranked categories: alive without deleterious events, alive with ≥1 deleterious event, and death.

4. Key Results

850 patients randomized (413 rapid AST, 437 standard).

Key dissociation: Despite dramatically faster diagnostic turnaround (ID: 2.7 vs 11.7h; AST: 13.5 vs 44.9h) and faster antibiotic modification, the primary patient-centered outcome (DOOR) was not significantly improved.

5. Internal Validity Assessment

• Randomization & Allocation: Multinational, multicenter randomization; specific concealment mechanics not detailed in available trial text.
• Protocol Adherence & Separation: Strong, well-documented diagnostic-speed separation (ID and AST turnaround times both highly statistically significant), confirming the intervention was delivered as intended — ruling out "the test wasn't actually faster" as an explanation for the null clinical result.
• Blinding & Detection Bias: Open-label; antimicrobial stewardship teams reviewed both arms, which is a methodological strength isolating the diagnostic (not stewardship) contribution, though open-label conduct could still introduce some performance bias.
• Missing Data & Sensitivity Analyses: ~20% of patients had organisms not covered by the rapid AST panel, potentially diluting the intervention's effect in the overall analysis (a coverage limitation rather than a trial-conduct flaw).
• Overall Internal Validity Conclusion: Moderate-to-strong — large (850-patient), multinational trial with confirmed, dramatic diagnostic-speed separation and a well-defined, patient-centered primary outcome; the null clinical result is therefore a genuine dissociation between diagnostic speed and patient-centered benefit, not an artifact of failed intervention delivery.

6. External Validity Assessment

• Population Representativeness: All-age (adults and children), multinational population with bloodstream infections — broadly representative of gram-negative bacteremia populations, specifically designed to include higher-antimicrobial-resistance regions.
• Practice Context: Differences across sites in resistance rates, clinical practices, and intervention implementation may have affected outcomes; ~20% organism-coverage gap for the rapid AST panel is a real-world platform limitation.
• Overall External Validity Conclusion: Moderate — multinational design with intentional high-resistance-region inclusion supports broad relevance, though site-level heterogeneity in resistance rates/practices and the rapid-AST panel's coverage gap should be considered when generalizing to a specific local context.

7. Strengths & Limitations

Strengths:

• Large (850-patient), multinational, all-age trial
• Patient-centered primary outcome (DOOR) rather than a simple process/timing metric
• Antimicrobial stewardship review in both arms, isolating the specific value of the diagnostic test itself
• Dramatic, statistically robust diagnostic-speed separation confirming intervention fidelity

Limitations:

• Open-label design
• ~20% of patients had organisms not covered by the rapid AST panel
• Site-level heterogeneity in resistance rates and clinical practices
• Evaluated rapid AST in combination with stewardship rather than as a fully independent, isolated variable

8. Interpretation & Practice Impact

• Clinical Implications: Rapid AST dramatically shortens diagnostic turnaround and antibiotic modification time (including faster escalation for resistant organisms), but this speed advantage does not translate into improved patient-centered outcomes (DOOR) at 30 days when stewardship review is already in place in both arms.
• Mechanistic Coherence: Consistent with a recurring pattern across prior rapid-AST trials (RAPIDS-GN and others) that generally show faster time-to-optimal-therapy without clearly reduced mortality or length of stay — diagnostic speed and hard clinical outcomes appear to be more loosely coupled than often assumed.
• Systems-Level Takeaway: The accompanying JAMA editorial (Srinivasan) argued the trial "should move hospitals firmly in the direction of implementing rapid" AST despite the neutral primary outcome, reflecting a view that faster, more targeted antibiotic therapy carries stewardship and antimicrobial-resistance-mitigation value beyond what a single 30-day DOOR endpoint captures.

9. Controversies & Subsequent Evidence

• Editorial Commentary: Arjun Srinivasan's accompanying JAMA editorial notably argues for rapid AST implementation despite the neutral primary outcome, emphasizing the "modest" but real diagnostic/stewardship benefits (faster escalation in resistant BSIs) that a single patient-centered composite may not fully capture — an important tension between statistical primary-outcome neutrality and broader clinical/stewardship value judgments.
• Guideline Integration: Contributes to an evolving evidence base (alongside a 2024 network meta-analysis and review on rapid diagnostic tests and stewardship programs for bloodstream infection) on the relative contribution of rapid diagnostics versus stewardship programs to improving clinical outcomes — an active area of debate in antimicrobial stewardship program design.

10. Summary & Executive Takeaway

Summary: FAST randomized 850 hospitalized patients (all ages, multinational) with gram-negative bacteremia to rapid AST plus standard testing or standard testing alone, with stewardship review in both arms. Rapid AST dramatically shortened diagnostic turnaround (ID: 2.7 vs 11.7h; AST: 13.5 vs 44.9h, both P<0.001) and time to antibiotic modification, but was not superior to standard testing on the primary patient-centered outcome (DOOR at 30 days).

Overall Takeaway: Rapid antimicrobial susceptibility testing delivers a dramatic, statistically unambiguous diagnostic speed advantage and faster antibiotic modification (especially escalation in resistant infections), but this did not translate into improved 30-day patient-centered outcomes when stewardship review was already standard in both arms — a genuine dissociation between diagnostic speed and measured clinical benefit that nonetheless has not deterred expert commentary from continuing to recommend implementation for its broader stewardship value.

11. Bibliography

• Banerjee R, Teng CB, Cunningham SA, et al. RAPIDS-GN. Clin Infect Dis. 2021;73(1):e39-e46.
• Peri AM, Chatfield MD, Ling W, et al. Rapid Diagnostic Tests and Antimicrobial Stewardship Programs for Bloodstream Infection: network meta-analysis. Clin Infect Dis. 2024;79(2):502-515.
• Srinivasan A. A Multipronged Approach [editorial]. JAMA. 2026;doi:10.1001/jama.2026.5504.