The Fluid Overload Ultrafiltration Percentage (FOUP): a novel metric that contextualizes fluid removal to initial burden in pediatric CRRT.

BACKGROUND: Optimal fluid removal during pediatric continuous renal replacement therapy (CRRT) remains challenging. Conventional metrics-net ultrafiltration rate (NUFR) and net fluid balance (NFB)-do not assess removal efficiency relative to initial fluid overload (FO). We hypothesized Fluid Overload Ultrafiltration Percentage (FOUP)-cumulative NFB normalized to initial FO-would reveal clinically meaningful patterns. METHODS: We studied 220 children receiving CRRT for ≥72 h. FOUP, NUFR, and NFB were calculated over three 24-h intervals. Subgroup analyses were performed within survivors stratified by FO (<5% vs. ≥5%), capillary leak index (CLI < 3.3 vs. ≥3.3), and body weight. RESULTS: Non-survivors (n = 84) had higher baseline FO (6.6% vs. 2.4%, p < 0.001) and received higher NUFR at all time points (all p < 0.01), yet achieved similar NFB and FOUP. NUFR trajectories diverged: survivors decreased (p = 0.003), and non-survivors increased (p = 0.013). In survivors, FOUP revealed a biphasic pattern in high-risk patients (FO ≥ 5% or CLI ≥ 3.3): vigorous early removal (-43.3% at 24 h) followed by a plateau (-2.0% to -4.1% at 48-72 h, p< 0.001), despite continued high NUFR. Hemodynamic stability was maintained. CONCLUSIONS: FOUP normalizes fluid removal to the initial FO, revealing a biphasic pattern in high-risk patients-early rapid removal followed by a plateau, signaling diminishing returns. These findings support a response-adaptive, FOUP-guided approach. IMPACT: This study introduces FOUP, a metric normalizing fluid removal to initial overload. Unlike weight-dependent NUFR or absolute NFB-which require baseline FO for interpretation-FOUP directly quantifies removal relative to initial burden. This enables FOUP to reveal clinically meaningful patterns: in patients with FO ≥ 5% or CLI ≥ 3.3, FOUP shows a biphasic trajectory-vigorous early removal followed by a plateau at 48-72 h-indicating that continued high-intensity ultrafiltration yields diminishing returns. By contextualizing removal to initial overload, FOUP provides a rational basis for response-adaptive fluid management.