The distinction at the heart of the debate is between functional and intrinsic AKI. A child whose kidney function has dropped mainly because of reduced renal perfusion may respond well to restoration of circulating volume and careful hemodynamic optimization. Apply the same fluid-driven approach to a child with established intrinsic kidney injury and accumulating fluid, however, and the intervention may do more harm than good. Unfortunately, telling these two situations apart early is far from straightforward, especially in infants and children, in whom baseline creatinine is often unknown, urine output can be difficult to measure accurately, and multiple mechanisms of injury frequently coexist in the same patient.

Writing in the same issue of Pediatric Research, Akash Deep of King’s College Hospital in London examines a new study by Feinauer and colleagues that tested whether urinary tissue inhibitor of metalloproteinases-2 (TIMP-2) and insulin-like growth factor-binding protein 7 (IGFBP7) could solve this diagnostic dilemma. The researchers studied 179 neonates and children, including 76 with AKI, of whom 22 were classified as having functional AKI and 54 as having intrinsic AKI. The product of the two urinary biomarker concentrations, written as [TIMP-2]×[IGFBP7], was compared against urinary calprotectin, fractional excretion of sodium (FENa) and the urinary protein-to-creatinine ratio. The results were revealing. The TIMP-2 and IGFBP7 product performed reasonably well at identifying AKI overall, with an area under the receiver operating characteristic curve (AUC) of 0.76, but its ability to separate functional from intrinsic AKI was considerably more modest, at an AUC of 0.68. Calprotectin did better, achieving an AUC of 0.85, while the humble urinary protein-to-creatinine ratio delivered the best discrimination of all, at 0.89. FENa, despite its long-standing clinical use for exactly this purpose, managed only 0.72.

Perhaps none of this should come as a surprise, because it reflects a fundamental mismatch between what these biomarkers were built to do and what the study asked of them. TIMP-2 and IGFBP7 are markers of tubular cell stress and cell-cycle arrest. They were developed principally to identify a kidney at risk of developing AKI, not to establish the anatomical or pathophysiological cause of injury that has already occurred. A stressed tubular cell is not necessarily an irreversibly damaged tubular cell, and cellular stress may appear during hemodynamic disturbance well before conventional evidence of intrinsic kidney injury shows up. Asking [TIMP-2]×[IGFBP7] to distinguish functional from structural AKI may therefore be asking the biomarker a fundamentally different question from the one it was designed to answer, and the modest AUC reflects that mismatch rather than a failure of the biomarker itself.

The distinction matters enormously as the field moves toward more precise phenotyping of AKI. Serum creatinine and urine output predominantly report on kidney function. TIMP-2 and IGFBP7 provide information about cellular stress. Other biomarkers, including neutrophil gelatinase-associated lipocalin (NGAL) and kidney injury molecule-1 (KIM-1), reflect different aspects of tubular injury, while calprotectin may offer a window into intrarenal inflammation. Proteinuria, though far less novel, can signal glomerular and tubular damage. Crucially, these markers are not competitors vying to replace one another; they measure different components of the broad syndrome that clinicians lump together under the single label of AKI. The search for a renal equivalent of troponin, the cardiac protein that revolutionized the diagnosis of heart attacks, has been seductive, but AKI is not one disease, and no single molecule is likely to capture its full complexity.

The new study also calls into question the traditional binary division of AKI into functional and intrinsic disease. This distinction is clinically useful, but kidney injury in critically ill children rarely obeys such tidy categories. AKI evolves over time: a child may begin with reduced renal perfusion and no significant structural injury, progress through a phase of tubular cellular stress, and, if the insult persists, go on to develop established tubular damage. In sepsis, cardiac disease and liver failure, hemodynamic, inflammatory and microcirculatory mechanisms may all operate simultaneously. At the bedside, functional and intrinsic AKI may therefore represent points along a continuum rather than two completely separate entities. Recent pediatric consensus recommendations have reached a similar conclusion, emphasizing AKI phenotyping and the need to move beyond a purely functional definition based on creatinine and urine output.

Timing, too, emerges as a critical variable. In the Feinauer study, urine samples were obtained only after AKI had already been diagnosed, and the biomarkers were measured at a single time point. The intrinsic AKI group also contained a higher proportion of patients with moderate-to-severe AKI when KDIGO stages 2 and 3 were considered together. Higher biomarker concentrations could therefore reflect, at least in part, the severity or duration of kidney injury at the moment of sampling rather than its original mechanism. Serial measurements beginning before, or very early in, the evolution of AKI would offer a far better picture of how stress and damage biomarkers change as functional impairment progresses to established injury, and could potentially allow clinicians to intervene during the reversible phase.

One finding in particular deserves attention, and it is the study’s most humbling. The urinary protein-to-creatinine ratio, a cheap and widely available investigation available in virtually every hospital laboratory, provided the best discrimination between functional and intrinsic AKI, outperforming every novel biomarker tested. Novelty, in other words, does not necessarily translate into greater clinical value. Before any new biomarker is introduced into routine practice, the field needs to demonstrate that it adds genuinely useful information to careful clinical assessment and inexpensive laboratory tests already in hand. That said, the commentary cautions against concluding that proteinuria alone can settle the question. The number of children with functional AKI in the study was small, the population ranged from neonates to adolescents, an age span over which renal physiology changes substantially, and several clinically important forms of functional AKI, including hepatorenal syndrome and congestive cardiorenal syndrome, were excluded. The definition of functional AKI was also necessarily retrospective, depending partly on response to fluid resuscitation or recovery of renal function within 72 hours. Although classification was performed independently by three clinicians with good agreement, the absence of a reliable reference standard remains one of the biggest obstacles facing AKI biomarker research.

The more promising path forward may lie in combining biomarkers with clinical context rather than using them in isolation. The Acute Disease Quality Initiative consensus has recommended pairing functional and damage biomarkers to improve diagnostic precision, and pediatric studies have shown that biomarker-defined phenotypes can provide prognostic information beyond serum creatinine alone. A prospective study of 134 critically ill children with AKI by Hui and colleagues illustrates the point vividly: urinary NGAL, TIMP-2, IGFBP7 and CCL14, assessed individually and serially, provided only moderate discrimination for persistent and prolonged AKI, but combining biomarker concentrations with the concurrent AKI stage substantially improved predictive performance. For bedside practice, that is the comparison that matters, because biological information appears to add the most value when interpreted alongside what clinicians already know about the patient, including underlying disease, hemodynamics, fluid balance, nephrotoxic exposure, creatinine trajectory and urine output.

The next step, the commentary argues, should be more than another round of AUC comparisons. Better discrimination is useful, but it does not by itself establish clinical utility. The decisive question is whether a biomarker result changes what clinicians actually do: does it influence fluid administration or de-resuscitation, nephrotoxin exposure, hemodynamic management or drug dosing? Can it identify children who need earlier nephrology involvement or closer follow-up after an episode of AKI? Ultimately, does biomarker-guided management improve kidney recovery or patient-centered outcomes? Prospective multicenter studies with serial measurements will be essential, describing how phenotype changes over time and whether the transition from stress to injury can be caught before serum creatinine betrays it. Some researchers have even proposed that biomarkers could eventually become theragnostic, not only identifying disease but helping to guide therapy as their concentrations shift in response to treatment. The study by Feinauer and colleagues is valuable not because it delivers a new stand-alone test for intrinsic AKI, but because it forces the field to be explicit about what it expects from a biomarker. Detecting kidney stress, demonstrating structural injury, measuring loss of function and identifying the cause of AKI are different tasks, and the future of AKI diagnosis will almost certainly involve combining clinical assessment, risk stratification tools, kidney function and selected markers of stress and damage. The challenge now is to determine which combinations genuinely change management and improve both short- and long-term outcomes for the child in front of us.

Subject of Research: Biomarkers for distinguishing functional from intrinsic acute kidney injury in critically ill children

Article Title: Beyond diagnosing acute kidney injury: defining what we need from kidney biomarkers

Article References: Deep, A. (2026). Beyond diagnosing acute kidney injury: defining what we need from kidney biomarkers. Pediatric Research. https://doi.org/10.1038/s41390-026-05540-0

Image Credits: AI Generated

DOI: 10.1038/s41390-026-05540-0

Keywords: acute kidney injury, biomarkers, TIMP-2, IGFBP7, calprotectin, serum creatinine, pediatric intensive care, proteinuria, NGAL, AKI phenotyping, tubular stress, precision medicine