Association of Admission Medication Regimen Complexity with Delirium in Critically Ill Adults: A Prospective Multicenter Cohort Study

Background: The medication regimen complexity–intensive care unit (MRC-ICU) score is a validated tool for assessing medication regimen complexity in critically ill patients and has been associated with several clinical outcomes. However, its association with delirium remains poorly characterized. This study aimed to evaluate the association between the day-1 MRC-ICU score and delirium development and to describe associated clinical outcomes. Methods: This prospective multicenter cohort study enrolled 364 adult patients admitted to intensive care units (ICUs) across four tertiary hospitals between January and April 2025. MRC-ICU, sequential organ failure assessment, and acute physiology and chronic health evaluation II (APACHE II) scores were recorded within 24 h of admission. The primary outcome was delirium during the ICU stay. Secondary outcomes were acute kidney injury (AKI), fluid overload (FO), renal replacement therapy (RRT), ICU and hospital mortality, and ICU and hospital length of stay. Logistic regression was used to assess the association between day-1 MRC-ICU score and delirium. Results: Delirium occurred in 12 patients (3.3%). Baseline MRC-ICU scores did not significantly differ between the delirium and no-delirium groups (median, 12 vs. 11; p = 0.364). No statistically significant association was detected per 5-point increase in the day-1 MRC-ICU score in either the unadjusted model (odds ratio [OR] 1.28, 95% confidence interval [CI] 0.73–2.23; p = 0.385) or the APACHE II-adjusted model (OR 1.27, 95% CI 0.73–2.22; p = 0.398). Patients who developed delirium had higher rates of incident AKI (33.3% vs. 4.5%; p = 0.009), FO (83.3% vs. 10.5%), RRT initiation (66.7% vs. 4.3%), ICU mortality (83.3% vs. 12.5%), and hospital mortality (100% vs. 17.0%) (p < 0.001 for the remaining outcomes). Conclusions: No statistically significant association was detected between the day-1 MRC-ICU score and delirium development. Delirium was associated with higher rates of adverse clinical outcomes. However, the small number of delirium events and imprecise estimates preclude definitive conclusions regarding the association between admission medication regimen complexity and delirium risk.