Direct versus automated antimicrobial susceptibility testing in positive blood cultures: a comparative study in a cancer care setting

Background: Bloodstream infections (BSIs) are a major cause of morbidity and mortality among cancer patients. Although automated antimicrobial susceptibility testing (AAST) is the reference method, its prolonged turnaround time (TAT) may delay appropriate antimicrobial therapy. Direct antimicrobial susceptibility testing (DAST) offers a rapid alternative. Objectives: To compare the accuracy and reliability of DAST with AAST, evaluate their turnaround time, and assess the potential clinical impact of earlier susceptibility reporting. Materials and Methods: This retrospective comparative study included 181 clinically significant bacterial isolates recovered from positive blood cultures processed between January and December 2024. DAST was performed directly from positively flagged blood culture bottles using the disk diffusion method, while identification and AAST were performed using the VITEK® 2 Compact system. Agreement, diagnostic performance, turnaround time, and potential clinical impact were analyzed. Results: Gram-negative bacilli accounted for 79.6% of isolates, with Escherichia coli (35.9%) being the predominant pathogen. Overall categorical agreement between DAST and AAST was 90.1%, with sensitivity, specificity, positive predictive value, negative predictive value, and diagnostic accuracy of 98.5%, 93.6%, 89.6%, 99.1%, and 95.4%, respectively (κ=0.814). Mean turnaround time was significantly shorter with DAST than AAST (17.4 ± 2.1 vs. 42.8 ± 5.7 hours; p<0.001), providing susceptibility results approximately 25 hours earlier. Earlier availability of DAST facilitated timely optimization of antimicrobial therapy. Conclusion: DAST demonstrated excellent agreement with automated AST while significantly reducing turnaround time. It is a reliable adjunct to automated susceptibility testing and may facilitate earlier targeted antimicrobial therapy and improved antimicrobial stewardship in cancer-care settings