Site-specific and time-varying Acinetobacter colonization and risk of invasive infection in critically ill patients

Purpose: Colonization by Acinetobacter baumannii is common among critically ill patients. However, the relative contribution of colonization at different anatomical sites and the impact of dynamic changes in colonization status over time remain incompletely defined. Methods: We conducted a retrospective cohort study of adult intensive care unit (ICU) patients with at least one surveillance culture positive for carbapenem-resistant Acinetobacter spp. (CRAB) between January 2022 and January 2026. Active surveillance cultures were obtained at ICU admission and weekly thereafter from respiratory, rectal, and skin sites. Progression to invasive CRAB infection (pneumonia and/or bloodstream infection) was assessed using multivariable logistic regression and Cox proportional hazards models with site-specific, time-updated colonization as exposures. Sensitivity analyses excluded infections occurring within 72 h of ICU admission. Results: Among 489 colonized ICU patients, 313 (64.0%) progressed to invasive CRAB infection during ICU stay. Respiratory colonization at ICU admission was strongly associated with progression to infection (adjusted OR, 5.46; 95% CI, 3.18–9.37), whereas rectal colonization was not independently associated with risk. In time-updated Cox models, respiratory (adjusted HR, 1.72; 95% CI, 1.35–2.18) and skin colonization (adjusted HR, 1.65; 95% CI, 1.31–2.08) were independently associated with infection risk. Colonization at multiple anatomical sites conferred additional risk. Conclusions: Progression to invasive infection was common and strongly associated with site-specific and time-varying colonization patterns, particularly involving the respiratory tract. Incorporating dynamic, multisite colonization data into risk assessment may improve identification of patients at highest risk for invasive infection and inform targeted infection prevention strategies in the ICU.