The problem begins with the diagnostic markers themselves. Serum creatinine, the workhorse of kidney assessment in adults and older children, is nearly useless in the first days of a newborn’s life because it largely reflects the mother’s creatinine, transferred across the placenta, rather than the infant’s own kidney function. It then fluctuates with muscle mass, fluid balance, and renal perfusion, all of which swing wildly in preterm infants. Urine output, the other pillar of AKI definitions, is notoriously difficult to measure accurately in babies weighing less than a kilogram, and many preterm infants paradoxically produce large volumes of urine during injury episodes. Clinicians are, in effect, navigating with broken instruments.

Enter neutrophil gelatinase-associated lipocalin, or NGAL. This 25-kilodalton protein, produced by neutrophils and by the tubular cells of the kidney itself, surges in the urine within hours of tubular injury—far earlier than creatinine can respond. In adult and pediatric populations, urinary NGAL has earned a solid reputation as an early warning system for AKI. But in preterm neonates, its promise has been throttled by a deceptively simple gap: nobody knows what normal actually looks like. Without well-defined reference ranges for healthy preterm infants, a uNGAL value is a number without a yardstick, and clinicians cannot tell whether a given concentration signals injury or simply reflects the biology of extreme prematurity.

A research team led by Nicole F. Asdell, Jonathan L. Slaughter, Elizabeth M. Bonachea, and Tahagod Mohamed of Nationwide Children’s Hospital in Columbus, Ohio, together with Shivam Joshi of Ohio State University, set out to build that yardstick. Drawing on archived urine samples from the Ohio Perinatal Research Network biorepository, collected between 2010 and 2020 at the hospital’s level IV neonatal intensive care unit, the researchers assembled a cohort of 100 infants born before 32 weeks of gestation. Crucially, these were babies who remained free of AKI, sepsis, necrotizing enterocolitis, urinary tract infection, and patent ductus arteriosus requiring treatment throughout their initial hospital admission—conditions known to elevate NGAL independently of kidney injury.

The exclusion criteria were applied with unusual rigor. Each infant’s medical record underwent an individualized chart review, with AKI defined by creatinine-only neonatal modified KDIGO criteria, urinary tract infection confirmed by treated positive urine cultures, and sepsis identified through antibiotic exposure beyond 48 hours, diagnostic problem lists, or positive blood cultures. This painstaking filtering was essential: because uNGAL rises in response to systemic inflammation and infection, even a single confounding illness would contaminate the normative data the study was designed to establish. Each infant contributed one urine sample, collected within the first seven days of life using sterile cotton balls placed in the diaper and stored at minus 80 degrees Celsius.

In the laboratory, the team quantified uNGAL concentrations using particle-enhanced immunoturbidimetry with BioPorto Diagnostics reagents on a Roche Cobas analyzer, and measured urinary creatinine enzymatically. Because both uNGAL and the uNGAL-to-creatinine ratio were heavily right-skewed, the researchers natural log-transformed the data before running correlations and regression models, and used both linear regression and LOESS smoothing to check for nonlinearity. The statistical architecture mattered: gestational age was analyzed in three prespecified bands—23 to 25 weeks, 26 to 28 weeks, and 29 to 31 weeks—with adjusted models incorporating infant sex and birth weight.

The results were striking in their consistency. Gestational age showed a significant inverse association with uNGAL: the earlier the baby was born, the higher the baseline concentration. The Pearson correlation coefficient between gestational age and uNGAL was minus 0.503, and it strengthened to minus 0.606 when the biomarker was normalized to urinary creatinine, with p-values below 0.0001 for every analysis. Birth weight told the same story, with correlations of minus 0.483 for raw uNGAL and minus 0.567 for the creatinine-normalized ratio. In the regression models, infants born at 23 to 25 weeks had dramatically higher uNGAL than those born at 29 to 31 weeks—a log-scale difference of 1.58 unadjusted and 1.32 after adjustment for birth weight and sex, both statistically robust.

Sex left its own independent imprint on the data. Female infants had significantly higher mean uNGAL concentrations than males—225 versus 165 nanograms per milliliter, a difference that persisted in multivariable models adjusting for gestational age and birth weight. The researchers point to several plausible mechanisms: sex-based differences in tubular enzyme expression, hormonal effects, and differences in urinary white blood cell excretion, including subclinical or asymptomatic leukocyturia, which previous work suggests is more prevalent in female infants. Earlier studies, including De Mul and colleagues’ reference value work in very low birth weight infants, had hinted at these patterns, but small samples and inconsistent methods had prevented the field from consolidating them into usable clinical thresholds.

Why would the youngest preterm babies carry the highest baseline NGAL? The authors favor an explanation rooted in developmental biology: immature tubular function, increased permeability of the glomerular filtration barrier, and physiologically elevated NGAL expression during active renal development all likely contribute. But they are careful to note that alternative factors may be at play in extremely preterm infants, including a heightened inflammatory milieu, subclinical infection or leukocyturia, and the medication exposures that are routine in this population—any of which could nudge uNGAL upward independent of structural immaturity. The study’s normalization strategy also adds credibility: by expressing NGAL relative to urinary creatinine, the researchers partially controlled for the dramatic fluctuations in fluid status and urine output that make raw concentrations unreliable in preterm infants.

The study is not without limitations, and the authors are candid about them. It was conducted at a single center, relied on spot urine samples rather than timed collections, and faced uncertainty about the long-term stability of frozen uNGAL specimens. Excluding infants with AKI, necrotizing enterocolitis, sepsis, and treated patent ductus arteriosus was necessary to establish normative data, but it narrows generalizability to the broader preterm population, where such conditions are common. Still, the core message stands: NGAL is detectable in the urine as early as 23 weeks of gestation, its concentration falls with increasing gestational age and birth weight, and it runs higher in female neonates. Any clinical interpretation of uNGAL must therefore be anchored to developmental maturity and sex.

The payoff, if subsequent studies validate these findings in larger and more diverse cohorts, could be substantial. A reliable, non-invasive, blood-sparing biomarker would allow clinicians to identify kidney injury in preterm infants before urine output changes or creatinine shifts become apparent—opening a window for timely interventions such as nephrotoxic drug adjustment, fluid optimization, and blood pressure support during the earliest, most treatable phase of injury. In a population where every milliliter of blood is precious and every day of undetected organ injury compounds lifelong risk, gestational age- and sex-specific reference ranges for uNGAL could transform one of neonatology’s most stubborn diagnostic blind spots into a manageable, measurable target.

Subject of Research: Establishing baseline urinary NGAL reference ranges for acute kidney injury detection in preterm neonates

Article Title: Urinary neutrophil gelatinase-associated lipocalin concentration: a baseline study for establishing reference ranges in preterm neonates

Article References: Asdell, N. F., Slaughter, J. L., Joshi, S., Bonachea, E. M., & Mohamed, T. (2026). Urinary neutrophil gelatinase-associated lipocalin concentration: a baseline study for establishing reference ranges in preterm neonates. Journal of Perinatology. https://doi.org/10.1038/s41372-026-02848-x

Image Credits: AI Generated

DOI: 10.1038/s41372-026-02848-x

Keywords: urinary NGAL, acute kidney injury, preterm neonates, biomarkers, serum creatinine, gestational age, birth weight, neonatal intensive care, kidney development, reference ranges, Journal of Perinatology, neonatology

Preterm infants face a unique physiological predicament. Born before their organs have completed development, they must maintain blood pressure, thermoregulation, respiration and digestion simultaneously, often while receiving intensive respiratory and circulatory support. The kidneys and the gastrointestinal tract are both highly oxygen-dependent organs, and both are vulnerable to injury when oxygen delivery falls short of demand. Necrotizing enterocolitis, a devastating bowel disease, and acute kidney injury are among the most feared complications of prematurity, and both have been linked in previous research to periods of compromised perfusion and tissue hypoxia.

The research team used near-infrared spectroscopy, a bedside optical technique that shines near-infrared light through tissue and measures how much is absorbed by oxygenated and deoxygenated hemoglobin, to track regional oxygen saturation continuously. Probes placed over the flank captured renal oxygenation, while probes over the abdomen monitored intestinal oxygenation. Because the technique is entirely non-invasive and painless, it can be applied repeatedly or continuously in the smallest and sickest infants without adding to their physiological burden, a major advantage over blood sampling or imaging approaches that offer only snapshots.

What makes the study particularly valuable is its paired design. Rather than examining kidney or gut oxygenation in isolation, the investigators measured both regions simultaneously in the same infants, allowing direct comparison of how the two vascular beds behave under identical systemic conditions. This matters because the splanchnic and renal circulations respond differently to stress. The gut is exquisitely sensitive to feeding, expanding its metabolic and oxygen demands with every milk bolus, while the kidney’s oxygen consumption is dominated by active sodium reabsorption and is influenced by medications, oxygen therapy and hemodynamic swings.

The study evaluated how feeding timing interacted with these oxygenation patterns. Feeding practices in neonatal intensive care units vary widely: some units begin minimal enteral feeds within hours of birth, others delay until the infant is clinically stable, and the pace of advancing feeds is a subject of ongoing debate. The reasoning behind cautious feeding is that a gut with marginal oxygen reserve may be poorly equipped to handle the postprandial surge in demand, potentially increasing the risk of feed intolerance or necrotizing enterocolitis. Yet prolonged delays in feeding carry their own costs, including gut atrophy, prolonged dependence on intravenous nutrition and increased risk of infection and liver injury.

By correlating continuous renal and intestinal oxygen saturation measurements with feeding episodes, the researchers were able to characterize how preterm infants’ regional oxygenation responds around feeds and whether the magnitude or pattern of those responses differs between the kidney and the gut. The paired measurements revealed that intestinal oxygenation behaves dynamically in relation to feeding, whereas renal oxygenation follows a more independent course shaped largely by systemic hemodynamics and clinical interventions. This dissociation underscores that a single global measure of oxygenation, such as arterial oxygen saturation, cannot capture the redistribution of blood flow that occurs between organ beds in the transitional period after preterm birth.

The clinical implications are potentially significant. If intestinal oxygenation measurements can identify infants whose guts are experiencing relative hypoxia before clinical signs such as feeding intolerance, abdominal distension or bloody stools appear, monitoring could shift from reactive to preventive. Neonatologists could, in principle, titrate the initiation and advancement of feeds against a real-time physiological signal, feeding more aggressively when intestinal oxygenation is robust and holding back when it signals distress. Similarly, renal oxygenation trends could serve as an early warning for acute kidney injury, which currently is often detected only after serum creatinine rises, a lagging marker that reflects injury after it has already occurred.

Like all physiological monitoring studies in this population, the work comes with methodological caveats. Near-infrared spectroscopy measures a regional venous-weighted saturation rather than direct tissue oxygen tension, and probe positioning, skin thickness and local edema can influence readings. The kidney’s retroperitoneal location and the interposition of bowel gas make renal signals technically more challenging to acquire than cerebral signals, and interpreting absolute values requires normative data across gestational ages and postnatal days. The authors’ paired comparison design helps address some of these concerns by using each infant as their own physiological control, isolating differences between organ beds from confounding by systemic factors.

Broader context strengthens the relevance of the findings. Research over the past two decades has established that low regional oxygenation in the first days of life is associated with adverse outcomes, including necrotizing enterocolitis, and that postnatal adaptation involves complex redistribution of cardiac output away from some vascular beds. Yet most monitoring studies have focused on the brain, leaving the abdomen comparatively underexplored despite the gut’s central role in neonatal morbidity. By bringing the kidney and the intestine into a single analytical frame and tying both to feeding, a modifiable care practice, this study adds a practical dimension to a growing physiological literature.

For now, near-infrared spectroscopy of the abdomen and kidney remains a research and select-care tool rather than routine standard practice, and no oxygenation threshold has been validated as a trigger for feeding decisions. But the trajectory of the field is clear: as continuous multi-organ monitoring becomes more feasible, the goal of individualized, physiologically guided nutrition in preterm infants moves closer to reality. The study’s paired demonstration that kidney and gut oxygenation diverge in the same infant at the same moment is a reminder that preterm physiology cannot be read from any single monitor, and that the organs we cannot see are often the ones most in need of a window.

Subject of Research: Simultaneous monitoring of renal and intestinal oxygenation in preterm neonates in relation to feeding timing

Article Title: Evaluation of kidney oxygenation compared to intestinal oxygenation and feeding timing in preterm neonates

Article References: Condit, P. E., Lindstrom, R., Gunderson, S. N., & Harer, M. W. (2026). Evaluation of kidney oxygenation compared to intestinal oxygenation and feeding timing in preterm neonates. Journal of Perinatology. https://doi.org/10.1038/s41372-026-02915-3

Image Credits: AI Generated

DOI: 10.1038/s41372-026-02915-3

Keywords: preterm neonates, near-infrared spectroscopy, kidney oxygenation, intestinal oxygenation, feeding timing, necrotizing enterocolitis, acute kidney injury, neonatal intensive care, regional oxygen saturation, enteral feeding, neonatal hemodynamics, Journal of Perinatology