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

ABSTRACT 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. Research in Context Cumulative fluid balance (CFB) is calculated by fluid (intake minus output) and weight measurements in pediatric ICU patients, but few studies have directly compared these methods across multiple institutions, unit types, and throughout PICU admission. CFB calculated using fluid measures consistently exceeded weight-based calculations, with divergence increasing later in admission and for patients with anchor weights recorded closer to PICU admissions and neonates. Divergence between CFB methods may reduce CFBf reliability later in the ICU stay, so accurate and frequent PICU weight recordings are essential for reliable fluid balance assessment. At the Bedside This multicenter study demonstrates a consistent positive divergence between CFB calculated by net fluid compared to weight-based calculations across diverse PICU and PCICU populations. Differences between methods diverged later in admission (e.g., after day 3), particularly among neonates and patients with earlier recorded anchor weights, while CFB calculated before the first unmeasured urine differed little from the overall cohort. Clinicians should understand these differences and the factors that influence them, prioritizing early and frequent patient weights throughout ICU admission to recognize when net fluid-based CFB diverges from a weight-based CFB.