Dynamic trajectories of serum sodium and urine output and their association with prognosis in sepsis: a group-based multi-trajectory modelling analysis using MIMIC-IV

Background: Serum sodium and urine output (UO) are routinely monitored indicators of electrolyte homeostasis, renal perfusion, and organ function in sepsis. However, the prognostic relevance of their combined longitudinal patterns remains unclear. Methods: We conducted a retrospective cohort study using the Medical Information Mart for Intensive Care IV database. Group-based multi-trajectory modelling identified combined trajectories of serum sodium and UO across four 24-hour periods during the first 96 hours after intensive care unit admission. Cox regression evaluated associations between trajectory subtypes and 30-day mortality, with subgroup and sensitivity analyses. Results: Among 7127 patients with sepsis, five trajectory subtypes were identified. Subtype 1 was characterised by a mild initial decrease in serum sodium followed by recovery and gradually increasing UO. Compared with subtype 1, subtype 2, characterised by mildly increasing serum sodium and gradually decreasing UO, showed the highest 30-day mortality risk in the fully adjusted model (HR = 1.85; 95% CI = 1.56-2.19, P < 0.001). Higher 30-day mortality was also observed for subtype 3, characterised by relatively stable serum sodium and UO that increased and then decreased (HR = 1.22; 95% CI = 1.03-1.46, P = 0.026); subtype 4, characterised by gradually increasing serum sodium and UO that increased before levelling off (HR = 1.31; 95% CI = 1.10-1.56, P = 0.003); and subtype 5, characterised by gradually decreasing serum sodium and UO that increased before levelling off (HR = 1.53; 95% CI = 1.29-1.82, P < 0.001). Subgroup and sensitivity analyses generally supported these findings. Conclusions: Five combined serum sodium and UO trajectory subtypes were identified in sepsis. Subtype 1 was associated with a favourable prognosis, whereas subtypes 2 and 5 carried the highest mortality risks. Early combined trajectories of serum sodium and UO may support prognostic risk stratification, although causal inferences cannot be drawn from this observational study.