Association of Fluid Balance Index with In-Hospital Mortality in Critically Ill Patients with Acute Pancreatitis: A Multicenter Retrospective Cohort Study

Fluid resuscitation is critical in managing acute pancreatitis (AP), yet both excessive and insufficient resuscitation can lead to poor outcomes. This multicenter retrospective cohort study investigated the relationship between the fluid balance index (FBI) and in-hospital mortality in critically ill AP patients. In total, 547 AP patients from the MIMIC-IV database and 91 patients from our center were classified into high-risk (FBI ≥ 145) and low-risk (FBI < 145) groups. The primary outcome was all-cause in-hospital mortality. The in-hospital mortality rate observed in the MIMIC-IV cohort was 8.96%. Machine learning analysis identified FBI as a key predictor of mortality. Multivariable Cox regression showed that patients with an FBI ≥ 145 had significantly increased in-hospital mortality (HR = 1.99, 95% CI 1.08–3.69) and 28-day mortality (HR = 2.90, 95% CI 1.50–5.64). Propensity score matching minimized baseline differences, and Kaplan-Meier survival analysis demonstrated higher mortality in the high-risk group ( p  < 0.05), consistent in our center’s cohort. Restricted cubic spline analysis revealed a near-linear increase in mortality with rising FBI ( p  < 0.05), confirmed by subgroup analysis. In conclusion, FBI is strongly associated with in-hospital mortality in critically ill AP patients, underscoring its potential to inform fluid resuscitation strategies.