Divergence Between Net Fluid and Weight-Based Evaluation in Calculating Cumulative Fluid Balance.

Objective: Although efforts have been made to standardize fluid balance calculations in the intensive care unit (ICU), there is a limited understanding of how different calculation methods relate to one another across an ICU admission. We quantified the agreement between the cumulative fluid balance calculated from fluid intake and output (CFBf) and from serial weights (CFBw) in critically ill children during the first week of ICU admission. Design: Retrospective, multicenter, federated observational study. Setting: Four pediatric medical-surgical ICUs (PICU) and two pediatric cardiac ICUs (PCICU) from four tertiary care centers. Patients: Analysis included 8,895 pediatric patients (<19 years old) representing 12,388 ICU encounters from 2023-2024. Interventions: None. Measurements and Main Results: A patient's anchor weight was the weight closest to ICU admission. CFBf and CFBw were calculated at the time of new weight measurements. We assessed agreement between CFBf and CFBw using Bland-Altman analyses, stratified by ICU day and patient subgroups (neonates, early anchor weights [weight on ICU day 0], and encounters with unmeasured urine occurrences). Across all units and subgroups, CFBf exceeded CFBw (mean difference: all patients = 4.7 %CFB, early anchor weight = 4.7 %CFB, neonates = 5.9 %CFB). The mean difference increased significantly over time (days 0-3: 2.7% vs. days 4-7: 8.1%, p<0.05), with greater divergence in neonates and those with early anchor weights. Conclusions: CFBf consistently exceeded CFBw across all subgroups, with a greater divergence on ICU days 4-7. Clinicians should understand these differences, prioritizing early and frequent patient weights throughout ICU admission. Future studies should assess each method's association with patient outcomes to identify the most clinically informative CFB method.