The metric, introduced by a team of researchers at the Children’s Hospital of Chongqing Medical University led by Dong Li, Xiaowei Xiong, Chunxiao Wang, Changyang Ye, Ke Bai, and Chengjun Liu, is called the Fluid Overload Ultrafiltration Percentage, or FOUP. The concept is mathematically straightforward: instead of expressing fluid removal in absolute terms or normalized to body weight, FOUP expresses the cumulative net fluid balance as a percentage of the child’s initial fluid overload—the excess fluid the patient carried at the start of therapy. In doing so, it reframes the question from how much fluid was removed to how much of the existing burden was eliminated. A child who begins treatment 10 percent fluid overloaded and loses 5 percent of that excess has, by this logic, achieved something qualitatively different from a child with minimal overload who loses the same absolute volume.

The rationale for this reframing comes from a growing body of evidence linking fluid overload to poor outcomes in pediatric critical care. Prior research, including systematic reviews and meta-analyses cited by the authors, has associated positive fluid balance with increased mortality in critically ill children, and studies of continuous renal replacement therapy have shown that the degree of fluid overload at initiation influences survival. Yet the conventional metrics used to titrate therapy do not account for the starting point. The net ultrafiltration rate is weight-dependent and says nothing about whether the removal is proportionate to the overload. The net fluid balance is an absolute number that similarly requires the clinician to mentally reference the baseline burden before it becomes interpretable. FOUP, the authors hypothesized, could make that contextualization automatic and reveal patterns that the older metrics obscure.

To test the hypothesis, the researchers analyzed 220 children who received continuous renal replacement therapy for at least 72 hours. For each patient, they calculated FOUP, net ultrafiltration rate, and net fluid balance across three consecutive 24-hour intervals, allowing them to track not just static values but trajectories over the crucial first three days of therapy. They then stratified the surviving patients into subgroups based on the severity of initial fluid overload, using a cutoff of 5 percent, and based on the capillary leak index, a composite marker of vascular permeability with a cutoff of 3.3, as well as by body weight. The capillary leak index matters because children with leaky capillaries distribute fluid differently, moving it from the bloodstream into tissues, which complicates both the assessment of overload and the hemodynamic tolerance of its removal.

The comparison between survivors and non-survivors produced a striking asymmetry. Among the 84 children who died, baseline fluid overload was substantially higher—6.6 percent compared with 2.4 percent in survivors, a difference that reached statistical significance. The non-survivors also received higher net ultrafiltration rates at every time point measured. Yet when the researchers looked at the net fluid balance and the FOUP achieved, the two groups were similar. In other words, the children who died were subjected to more aggressive removal per kilogram of body weight but did not end up with proportionally greater reduction of their fluid burden, a discrepancy that the conventional metrics alone could not have exposed. The trajectories of the ultrafiltration rate also diverged in a telling way: survivors showed a declining rate over the study period, while non-survivors showed a rising one, and both trends were statistically significant.

The most clinically provocative finding emerged from the subgroup analysis of survivors who were at high risk—those with initial fluid overload of 5 percent or greater, or a capillary leak index of 3.3 or higher. In these patients, FOUP traced a distinctly biphasic pattern. In the first 24 hours, fluid removal was vigorous, with FOUP reaching approximately minus 43.3 percent, meaning nearly half of the initial fluid excess was cleared within a single day. But over the subsequent 48 to 72 hours, the curve flattened dramatically, with FOUP changing by only 2.0 to 4.1 percent, a plateau that was highly significant statistically. Crucially, this plateau occurred despite the continued delivery of high ultrafiltration rates. The machine kept pulling fluid, but the proportional reduction of the overload stalled—a signal, the authors argue, of diminishing returns.

Why would continued high-intensity ultrafiltration stop translating into proportional fluid burden reduction? The study’s data offer a clue in the hemodynamic figures. Throughout the first 72 hours of therapy, the high-risk survivors maintained hemodynamic stability, which the authors documented in their analysis of cardiovascular parameters. This suggests that the plateau was not simply a consequence of clinicians backing off to prevent blood pressure crashes. Rather, it points toward the physiology of fluid distribution: in patients with significant capillary leak, fluid removed from the intravascular space may be continuously replenished from the interstitial compartment, so that the net proportional clearance of the total overload lags far behind the raw ultrafiltration volume. FOUP, by normalizing to the initial burden, makes this decoupling visible in a way that a stable ultrafiltration rate does not.

The practical implication the authors draw from these patterns is a shift toward response-adaptive fluid management. Under a FOUP-guided framework, which the team sketches as an iterative cycle in their published figures, the intensity of ultrafiltration would be titrated not against a fixed weight-based target but against the measured response of the fluid burden itself. When FOUP shows rapid proportional clearance, as it did in the first 24 hours of high-risk patients, aggressive removal appears productive. When FOUP plateaus, as it did at 48 to 72 hours, the data suggest that pushing the ultrafiltration rate higher yields little additional proportional benefit while potentially increasing the risk of hemodynamic instability. The metric thus offers a physiological rationale for easing off, rather than an arbitrary protocol threshold.

The study carries the usual caveats of retrospective observational research. It was conducted at a single institution, the Children’s Hospital affiliated with Chongqing Medical University, with ethics approval and a waiver of individual informed consent given the retrospective design, and the analysis reflects the practice patterns of one center. The biphasic pattern describes an association observed in survivors of high-risk subgroups; it does not by itself prove that tapering ultrafiltration after the plateau improves outcomes. That question would require prospective testing, ideally in a randomized framework comparing FOUP-guided management with conventional rate- or balance-based protocols. The authors’ funding came from a Chongqing regional medical research project, and they report no competing interests.

Even so, the conceptual contribution is likely to resonate beyond pediatrics. Fluid management on continuous kidney replacement therapy remains one of the most contested territories in intensive care, with trials in adults exploring protocolized versus monitoring-driven approaches and observational studies documenting wide practice variation. FOUP joins a family of normalized indices—such as percent fluid overload itself and the capillary leak index—that attempt to convert raw volumes into physiologically meaningful proportions. Its distinctive move is to normalize the removal, not just the burden, creating a ratio that is interpretable without reference to body weight or baseline volume status. If prospective studies confirm that the biphasic trajectory holds across centers and that response-adaptive titration improves survival or reduces complications, a simple percentage could reshape how intensivists think about one of their most delicate daily decisions: not how much fluid to take out, but how much of the burden remains to be taken.

Subject of Research: A novel fluid removal metric (FOUP) for pediatric continuous renal replacement therapy

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

Article References: The Fluid Overload Ultrafiltration Percentage (FOUP): a novel metric that contextualizes fluid removal to initial burden in pediatric CRRT. (n.d.). https://doi.org/10.1038/s41390-026-05469-4

Image Credits: AI Generated

DOI: 10.1038/s41390-026-05469-4

Keywords: FOUP, pediatric CRRT, fluid overload, ultrafiltration, net fluid balance, capillary leak index, critical care, acute kidney injury, fluid management, hemodynamics, Pediatric Research, intensive care