Early targeted therapy guided by rapid phenotypic antimicrobial susceptibility testing in critically ill patients with gram-negative bacterial bloodstream infections: a retrospective cohort study

Background Early administration of appropriate antimicrobial therapy is crucial for survival in critically ill patients with Gram-negative bacterial bloodstream infections (GNB BSIs). Rapid phenotypic antimicrobial susceptibility testing (RAST) can provide faster susceptibility results, enabling timely optimization of therapy. This study evaluates the real-world clinical impact of RAST versus conventional AST in improving early antimicrobial treatment and clinical outcomes in an intensive care unit (ICU) setting with a high burden of multidrug-resistant (MDR) pathogens. Methods We conducted a retrospective, observational study in the ICU of ‘Renato Dulbecco’ University Hospital (Catanzaro, Italy) from April 1st to September 30th, 2025. Consecutive adult patients (≥18 years) with monomicrobial GNB BSIs were included and divided into two groups based on the susceptibility testing strategy used: RAST or conventional AST. The primary outcome was 30-day all-cause mortality. Secondary outcomes included administration of early active antimicrobial therapy (i.e., administration of at least one active agent in vitro within 24 h of blood culture sampling), clinical failure, length of both ICU and total hospital stay. Results A total of 133 patients were included (RAST: 37; AST: 96). Thirty-day mortality was observed in 4/37 patients (10.8%) in the RAST group and in 30/96 (31.3%) in the AST group (P= 0.015). Early active therapy was administered to 32/37 (86.5%) RAST patients versus 44/96 (45.8%) in the AST group (P< 0.001). Clinical failure occurred in 0/37 RAST patients versus 20/96 (20.8%) AST patients (P= 0.003). Mean ICU stay was 30.3 ± 22.9 days (RAST) versus 36.9 ± 25.6 days (AST), P= 0.17. At Cox regression analysis, RAST-guided management (HR = 0.16, 95% CI 0.04–0.62) and early active therapy (HR = 0.51, 95% CI 0.19–0.94) were independently associated with survival (Figure 1). Conclusions RAST significantly increased the likelihood of early active therapy and was associated with reductions in both 30-day mortality and clinical failure in critically ill patients with GNB BSIs. These findings suggest that RAST may represent a valuable tool to optimize antimicrobial therapy in ICU settings, particularly in light of the increasing prevalence of MDR GNB.Figure 1.Cox regression analysis of factors associated with clinical failure in Gram-negative bloodstream infections (GNB BSIs). Early active in vitro therapy was defined as the administration of at least one active agent in vitro within 24 h of blood culture sampling. RAST, rapid phenotypic antimicrobial susceptibility testing.