Clinical Characteristics, Antimicrobial Susceptibility, Treatment, and Outcomes of Achromobacter Bacteremia: A 20-Year Retrospective Cohort Study

Background/Objectives: Achromobacter bacteremia is uncommon, difficult to treat and lacks historically consistent organism-specific susceptibility criteria. We described its clinical characteristics, antimicrobial exposure, susceptibility patterns, treatment, and outcomes over 20 years. Methods: We retrospectively reviewed adults with Achromobacter-positive blood cultures at a Thai tertiary-care hospital during 2006–2025. The first eligible episode per patient was used for patient-level analyses. Historical susceptibility categories were retained as reported. Available matched blood-isolate minimum inhibitory concentrations (MICs) were retrospectively interpreted using Clinical and Laboratory Standards Institute (CLSI) M100, 36th edition. Results: Forty-nine eligible episodes occurred in 47 patients, of which 43/47 (91.5%) were nosocomial. Prior systemic antibiotics and carbapenems had been received by 43/45 (95.6%) and 26/45 (57.8%) patients, respectively. Prior carbapenem exposure was associated with meropenem non-susceptibility (50.0% vs. 11.1%; odds ratio (OR) 7.64, p = 0.010). Thirty-day mortality was 23/46 (50.0%). Non-survivors had higher median National Early Warning Score (NEWS) than survivors (6 vs. 3; p = 0.005). Mortality was lower among patients receiving active than inactive empiric therapy (39.4% vs. 76.9%; OR 0.20, p = 0.047), but the association attenuated after adjustment for vasopressor use. Historical disk and contemporary MIC categories were not fully concordant in the small matched subset (n = 7). Conclusions: Achromobacter bacteremia occurred predominantly as a nosocomial infection in heavily antibiotic-exposed patients and was associated with high mortality. Prior carbapenem exposure was associated with meropenem non-susceptibility, while mortality was closely related to acute illness severity. Historical susceptibility results should be interpreted in the context of the testing method and breakpoint framework.