Microbiology Laboratory Testing Practices of Gram-Negative Bloodstream Infections With Difficult-to-Treat Resistant Phenotypes in US Hospitals

Abstract Background Rapid administration of effective antimicrobial therapy is crucial for managing bloodstream infections (BSIs), especially those caused by pathogens with difficult-to-treat resistance (DTR; ie, resistance to all first-line antimicrobials). The purpose of this study is to describe the epidemiology and time course of microbiological testing in US laboratories for gram-negative BSIs (GN-BSIs). Methods A retrospective observational cohort study of adults with GN-BSI between 1 January 2017 and 30 June 2023 was conducted using a large, geographically diverse, hospital-based administrative US database. Antimicrobial susceptibility testing (AST) reporting and time to results of traditional and next-generation antimicrobials (ceftazidime-avibactam, ceftolozane-tazobactam, cefiderocol, meropenem-vaborbactam, imipenem-relebactam) were evaluated according to the isolates’ resistance phenotype. Results A total of 110 322 GN-BSI episodes were included, of which 0.6% exhibited a DTR phenotype. Among all episodes, the average time to results for AST was 73.5 ± 26.7 hours from blood culture collection. The average time to AST results was longer for DTR (92.2 ± 27.8 hours) and carbapenem not-susceptible phenotypes (90.1 ± 28.0 hours) than for nonresistant phenotypes (72.9 ± 26.7 hours). Overall, 2.5% of GN-BSI episodes had AST reported for at least 1 next-generation antimicrobial, increasing from 0.2% in 2017 to 7.7% in 2023. Among patients with isolates showing DTR phenotypes, 30.5% had AST reported for at least 1 next-generation antimicrobial. Conclusions Time to AST results for next-generation antimicrobials and antibiotic-resistant organisms is prolonged in US laboratories for blood cultures with gram-negative organisms. Testing frequency and timing of AST reporting for next-generation antimicrobials likely contribute to their clinical utilization.