Prognostic Impact of Cumulative Fluid Balance in Critically Ill Patients: A Retrospective Cohort Study

Background Cumulative fluid balance (CFB) reflects net fluid accumulation during intensive care unit (ICU) stay. However, prognostically relevant thresholds at ICU discharge remain insufficiently characterised. This study aimed to evaluate the association between CFB measured at ICU discharge and clinical outcomes in critically ill patients. Methods We conducted a retrospective single-centre cohort study including adult critically ill patients admitted to a tertiary mixed ICU between September 2022 and September 2023. CFB was calculated as the net difference between total fluid input and output during ICU stay and expressed as a percentage of baseline body weight at ICU discharge. Predefined CFB thresholds (>7%, -3% to 7%, and <-3%) were selected to reflect clinically relevant degrees of fluid accumulation and depletion. The primary outcome was ICU mortality. Secondary outcomes included the requirement for FiO₂ >0.60, duration of invasive mechanical ventilation, increase in serum creatinine >50% from baseline, need for renal replacement therapy (RRT), and length of stay. Results A total of 93 patients were analysed. ICU mortality differed significantly across CFB groups, being highest in patients with CFB >7% (n=10, 33.3%) compared with -3% to 7% (n=5, 14.3%) and <-3% (n=2, 7.1%) (p = 0.027). Requirement for FiO₂ >0.60 was more frequent in the CFB >7% group (n=13, 43.3%; p = 0.007), and median duration of mechanical ventilation was longer in this group (11 days, IQR 6.1-12.3; p = 0.029). Patients with CFB <-3% demonstrated numerically higher rates of creatinine increase >50% (n=8, 28.6%) and RRT (n=5, 17.9%), although these differences were not statistically significant. Hospital mortality and length of stay did not differ significantly between groups. Baseline severity scores were comparable across groups. Conclusions CFB at ICU discharge was associated with distinct prognostic patterns in critically ill patients. A markedly positive CFB (>7%) was associated with higher ICU mortality and greater respiratory support requirements, whereas a markedly negative CFB (<-3%) showed a trend towards less favourable renal outcomes. These findings suggest that CFB may serve as a complementary prognostic marker in the later phases of critical illness, although causal inferences cannot be established from this retrospective analysis.