Risk factors, prognostic impact, and antimicrobial susceptibility profiles of carbapenem-resistant Acinetobacter baumannii bloodstream infections: a retrospective cohort study

This study aimed to identify the independent risk factors for bloodstream infections (BSI) caused by carbapenem-resistant Acinetobacter baumannii (CRAB). We also sought to evaluate its prognostic impact on 28-day all-cause mortality, and to systematically analyze the antimicrobial susceptibility profiles of CRAB strains and the clinical outcomes of different treatment regimens. We conducted a single-center retrospective cohort study involving patients with A. baumannii BSI admitted to a tertiary teaching hospital from January 2023 to December 2024. Of 260 initially screened patients, 38 with polymicrobial infections were excluded per predefined criteria, resulting in a final cohort of 222 adults with confirmed monomicrobial A. baumannii BSI. Based on their susceptibility results, patients were divided into a CRAB group (n = 22) and a carbapenem-susceptible A. baumannii (CSAB) group (n = 200). We extracted clinical and microbiological data and used the Mann–Whitney U test and χ² test for intergroup comparisons. Independent risk factors were identified via multivariate logistic regression. Furthermore, Kaplan–Meier curves and Cox proportional hazards models were used to assess the independent prognostic value of CRAB infection on 28-day mortality. Antimicrobial susceptibility testing (AST) was performed using the VITEK 2 automated microbiology system. The interpretation of susceptibility results primarily followed the 2023 Clinical and Laboratory Standards Institute (CLSI) guidelines, whereas tigecycline susceptibility was interpreted according to the 2024 European Committee on Antimicrobial Susceptibility Testing (EUCAST) standards. The CRAB detection rate in our study was only 9.91% (22/222). This rate is noticeably lower than the national average, and occurred concurrently with the rollout of our hospital’s tiered carbapenem management program (during which carbapenem usage density dropped from 78 to 42 DDD/100 bed-days). Multivariate analysis showed that prior carbapenem exposure (OR = 4.77, 95% CI 1.60–14.03, P = 0.004), underlying malignancy (OR = 3.04, 95% CI 1.06–8.40, P = 0.033), and age ≥ 65 years (OR = 3.58, 95% CI 1.37–10.22, P = 0.012) were independent risk factors for CRAB BSI. The 28-day all-cause mortality in the CRAB group was 45.45%, which was significantly higher than the 19.00% observed in the CSAB group (P = 0.010). Even after adjusting for confounders, CRAB infection remained an independent predictor of 28-day mortality (adjusted HR = 2.43, 95% CI 1.14–5.16, P = 0.021).Regarding susceptibility, based on clinical stratification, polymyxin B showed the highest clinical susceptibility rate (81.82%) against CRAB. When used as monotherapy (based on ideal body weight IBW: 1.5–2.0 mg/kg, IV infused every 12 h), polymyxin B yielded a clinical response rate of 71.43% and a 28-day mortality rate of 28.57%. Any associated nephrotoxicity was fully reversible with prompt intervention. Advanced age, underlying malignancies, and prior carbapenem exposure are independent risk factors for CRAB infection in patients with A. baumannii BSI, significantly elevating the risk of short-term mortality. Polymyxin B exhibits excellent in vitro activity against CRAB strains isolated from our center. However, in light of the small size of the CRAB subgroup (n = 22), the imbalance between the CRAB and CSAB groups, and the inherent risk of confounding by indication in this single-center retrospective observational design, any inferences regarding its clinical efficacy must be made with extreme caution; definitive clinical recommendations require further validation in large-scale, prospective randomized controlled trials. Furthermore, the decline in CRAB detection rates occurred alongside reduced carbapenem consumption; this is only a temporal association, and no causal inferences can be drawn from this observational study.