Association between serum osmolality and delirium in critically ill patients: a retrospective cohort study

Abstract Background Delirium is a common and serious neuropsychiatric disorder in intensive care unit patients. It is associated with adverse outcomes and long‐term sequelae. Serum osmolality is a simple‐to‐calculate, readily available physiological parameter that may influence neurological function. Its relationship with delirium remains unclear. Aims This retrospective study aimed to evaluate the association between serum osmolality and delirium in a critically ill population. Methods We conducted a retrospective cohort study of 30 073 patients screened from the Medical Information Mart for Intensive Care IV database. Serum osmolality was calculated and categorised into hypo‐osmolality, normal osmolality and hyper‐osmolality groups. The primary outcome was delirium. Kaplan–Meier curves, Cox proportional hazards models and restricted cubic splines were used to examine the association. Sensitivity analyses, competing risk models and subgroup analyses were performed. Results Among 30 073 eligible patients, 2.9% had hypo‐osmolality, 82.1% normal osmolality and 15.1% hyper‐osmolality. In the fully adjusted model (Model III), hyper‐osmolality was associated with a 35% increased risk of delirium (HR = 1.35, 95% CI: 1.27–1.45, P < 0.001). Hypo‐osmolality was associated with a 24% increased risk of delirium (HR = 1.24, 95% CI: 1.05–1.46, P = 0.013). Restricted cubic spline analysis revealed a U‐shaped relationship ( P < 0.001), with the lowest risk at an osmolality of 293.2 mOsm/kg. Results remained robust in sensitivity and competing risk analyses. Subgroup analyses showed stronger associations in patients receiving mechanical ventilation or vasopressors. Conclusions Both hyper‐osmolality and hypo‐osmolality are associated with an increased risk of delirium in critically ill patients. Maintaining osmolality homeostasis may be important for delirium prevention, especially in those receiving mechanical ventilation or vasopressors.