Primary nephrotic syndrome (PNS) is among the most common chronic kidney diseases affecting children, and the magnitude and pattern of proteinuria remain central to diagnosis, risk evaluation, treatment selection and monitoring of response (1,2). As is known, 24-h urine protein (24hUP) excretion has long been the gold standard for quantifying protein loss and is a strong predictor of renal outcomes across glomerular diseases, underscoring the clinical need for accurate 24hUP estimates in routine care, beyond diagnosis alone (1,2). Current guidelines permit the diagnosis of nephrotic-range proteinuria using either a urine protein-creatinine ratio (UPCR) ≥200 mg/mmol or a 24hUP ≥40 mg/m²/h (2,3). However, these diagnostic cut-offs primarily establish disease presence and do not provide the granularity required for decisions, such as initiating thromboprophylaxis, triaging for inpatient monitoring, or tailoring follow-up intensity during the acute phase (3,4).

In pediatrics, 24-h urine collections are frequently impractical due to poor cooperation, incomplete or contaminated samples, and the logistical burden on families and health systems, which compromises reliability and feasibility in routine practice (5). A single-voided UPCR is therefore widely recommended as an accessible surrogate, physiologically normalizing protein to creatinine and approximating 24hUP (2,3). Although pediatric studies have demonstrated a positive association between UPCR and 24hUP, residual heterogeneity persists due to age- and sex-related differences in creatinine generation, edema and hydration status, sampling time, assay methodology and laboratory quality control (6,7). Moreover, the association between UPCR and 24hUP is often multiplicative and heteroscedastic, predisposing to proportional bias at the lower and upper extremes on the raw scale and limiting confidence in directly inferring very high proteinuria from a single UPCR value, precisely when clinicians seek to avoid missing high-risk presentations (8,9). Population-specific kidney equations have recently been derived in Vietnamese patients, underscoring that nephrology prediction tools often require local calibration rather than direct adoption of Western formulas (10).

Key evidence gaps include the lack of externally validated UPCR-to-24hUP equations in pediatric PNS, the limited assessment of clinical interchangeability beyond simple correlation and the sparse representation of Asian cohorts. The present study aimed to: i) Derive a pediatric UPCR-to-24hUP equation in PNS; ii) externally validate its predictive performance, including calibration and agreement; and iii) translate predictions into clinically actionable gray-zone thresholds, using prediction-interval (PI)-guided classification and decision-curve analysis (DCA) to compare net clinical benefit with threshold-based care.

Severe PNS is critical clinically not only due to massive edema and profound hypoalbuminemia, but also due to early, potentially life-threatening complications such as thromboembolism, severe bacterial infection/sepsis and intravascular volume depletion with prerenal acute kidney injury, all of which cluster in the high-proteinuria, hypoalbuminemia phase and therefore track with the severity of proteinuria (15-18). Yet, there is currently no widely adopted pediatric tool that turns a single-void UPCR into a reliable estimate of severity tier of 24hUP to support those specific bedside decisions. UPCR correlates well with 24hUP in children; however, key gaps persist. The majority of studies stop at simple correlations and do not assess agreement, proportional bias, or calibration across the full proteinuria range (6,19,20). Numerous equations also lack external validation in independent pediatric nephrotic cohorts, particularly in Asian children, limiting use where 24-h urine collection is impractical. In addition, the UPCR-24hUP association is multiplicative (log-log), causing heteroscedastic and proportional error on the raw scale, with underestimation at the highest protein leaks, exactly where clinicians most need to identify children at highest risk (8,20,21).

In the present study cohort of children with PNS, a multivariable equation was developed that converts a single-void UPCR into an estimated 24-h protein excretion. Herein, three creatinine-excretion approaches (Cockcroft-Gault, Hellerstein, and Ghazali-Barratt) were compared to standardize UPCR for body-size variability. The Cockcroft-Gault-based approach provided the optimal overall agreement and the most favorable accuracy-feasibility balance in the cohort; alternative pediatric formulas did not improve performance and were therefore not retained. The final model (M₆) used a Cockcroft-Gault-based adjusted UPCR multiplied by an age and sex correction, reflecting physiologic variation in creatinine generation, muscle mass and urine concentration across childhood and adolescence, consistent with prior studies (22-25). This approach is motivated by the known limitations of ‘raw’ UPCR: Protein and creatinine do not scale identically with growth, edema, hydration status and lean body mass, which introduces heteroscedastic error and proportional bias, particularly at the extremes of protein loss (8,21,26).

This adjustment also aligns with the broader goal of reducing body-size-related variability when interpreting proteinuria metrics in children. The fractional excretion of total protein (FETP), the ratio of total protein clearance to creatinine clearance, has been proposed as a physiologically grounded, clearance-based index to account for body size with preserved kidney function (27). However, FETP typically requires synchronous blood measurements in addition to urine protein and urine creatinine, which were not consistently available at the time of urine sampling in the dataset in the present study; thus, calculating FETP would have introduced substantial missingness and potential selection bias. By contrast, UPCR is emphasized in the majority of pediatric nephrology guides as a practical alternative to timed urine collections for defining and monitoring nephrotic-range proteinuria, supporting our focus on UPCR for real-world, guideline-concordant implementation (1-3). Future studies with standardized paired blood-urine sampling are warranted to directly compare UPCR-based equations with clearance-based indices such as FETP to assess any incremental predictive or clinical utility.

The model used herein explained a large proportion of variability in 24hUP during development and performed robustly in a temporally independent validation cohort without refitting. In external validation, it achieved a high coefficient of determination (R²=0.91), low absolute prediction error (9 mg/m²/h) and clinically tight accuracy (94% within 20 and 96% within 30% of observed 24hUP). Calibration remained acceptable when the original coefficients were applied unaltered, and the average bias on the original mg/m²/h scale was minimal. These findings suggest that a single-voided urine specimen can recover the approximate 24-h protein excretion of a child with clinically usable precision, not merely correlation.

The present study also examined how the model behaves at the most critical areas clinically: At the extreme upper end of proteinuria (24,28). Bland-Altman analysis in the validation cohort revealed narrow LoA and only modest mean bias overall, but with proportional error at very high 24hUP values; specifically, the model tended to slightly underpredict the most extreme protein losses. This matters clinically as children with extremely high-grade proteinuria have the highest short-term risk for complications, such as intravascular volume depletion, severe hypoalbuminemia requiring infusion, bacterial infection and thromboembolism in the setting of marked urinary protein wasting and hemoconcentration (15,16,18). These are the very patients for whom pediatric nephrologists decide on immediate inpatient monitoring and aggressive supportive care. As proportional bias emerges at the extreme end of proteinuria, it may be unsafe to base high-stakes decisions on a single predicted value alone. Accordingly, when predicted values fall in the highest range, confirmatory testing with a timed urine collection (full 24-h urine) should be considered to support clinical decision-making. In such children, escalation decisions should incorporate the uncertainty of the model, not only its mean prediction.

To address this issue, in the present study, model output was operationalized into a three-zone decision rule based on the PI of each child, rather than relying solely on the point estimate. Of note, two clinically relevant thresholds were prespecified for 24hUP (100 mg/m²/h as a lower-risk cut-point, and 150 mg/m²/h as a higher-risk cut-point) consistent with levels at which clinicians begin to escalate surveillance, consider albumin/diuretic management, or are concerned about the thrombotic risk (1-3,29). For each patient, the following questions were raised: Even in a worst-plausible scenario consistent with model error (95% PI), is this child clearly below the cut-point, clearly above it, or uncertain (‘gray zone’)? Using this PI-based strategy at the 150 mg/m²/h threshold, a full 24-h urine collection could have been avoided in 75% of children in the validation cohort, while directing only 1% straight to ‘must collect now’, and leaving 24% in a gray zone for clinician review, without falsely ruling out any truly very-high-proteinuria cases (0% false-omits). At the 100 mg/m²/h threshold, the same framework again separated ‘safe to defer collection’, ‘must collect’, and ‘gray zone’ with no missed high-proteinuria cases. In practice, gray-zone results (i.e., the PI overlaps the cut-point) are most effectively handled with an iterative confirmation strategy rather than immediate escalation. The authors suggest the following: i) Repeating a spot UPCR after stabilization (e.g., next void or within 12-24 h) to reduce random variability; ii) integrating contemporaneous clinical context (edema burden, hemodynamics, urine output) and key laboratory tests (serum albumin and creatinine/eGFR); and iii) obtaining confirmatory timed urine collection (full 24-h urine) when uncertainty persists or management decisions are high-stakes. Conceptually, this functions as a triage layer on top of the numeric prediction: It concentrates limited nursing and laboratory effort on the few children whose proteinuria is most concerning, and it reassures clinicians that avoiding immediate 24-h collection in the others is unlikely to miss a dangerous presentation.

This approach is responsive to how pediatric nephrotic syndrome is actually managed. A tool that i) estimates the likely magnitude of ongoing protein loss from a single spot sample and ii) classifies the child into ‘clearly below’, ‘clearly above’, or ‘uncertain relative to clinically relevant danger zones’, directly mirrors bedside thinking. DCA in the external validation cohort supports this interpretation: Across plausible intervention thresholds (reflecting the tolerance of the clinician to act on suspected very high proteinuria), a model-guided strategy provided higher net clinical benefit than treating all children as high risk (i.e. obtaining 24-h collections on everyone) or treating none as high-risk. This indicates that the model is not only statistically well calibrated, but also improves how efficiently attention and resources would be allocated. This ‘equation → triage rule → net benefit’ chain is largely absent from prior pediatric work in nephrotic syndrome and, to the best of our knowledge, has not yet been reported in an Asian pediatric cohort.

The present study has several strengths. It uses consecutive real-world pediatric nephrotic admissions, including children with substantial hypoalbuminemia and heavy protein loss, rather than an artificially ‘clean’ subset. The validation cohort was acquired later in time, reducing optimism from overfitting. The present study reports discrimination-like metrics (R², P20/P30), absolute error (MAE, RMSE), calibration (slope/intercept), clinical agreement on the original scale and decision safety at actionable thresholds. This prediction model study is reported in accordance with the TRIPOD statement (30). Of note, the PI-based gray-zone rule achieved 0% false-omits for very high proteinuria at 150 mg/m²/h, while suggesting that the majority of children could safely avoid a 24-h urine collection. That directly targets the operational bottleneck in pediatric nephrotic care: Deciding who truly needs an immediate timed collection as opposed to who can be managed using spot data plus close observation.

However, certain limitations of the present study should also be acknowledged. First, although the temporal split provides external validation, both cohorts were drawn from a single center. Accordingly, external validity is limited, and the equation should be externally validated in independent, multicenter and multi-ethnic pediatric cohorts prior to widespread clinical adoption. Variation in anthropometry, nutritional status, steroid exposure, supportive care practices (e.g., albumin infusion thresholds, thromboprophylaxis norms), and laboratory calibration across institutions may affect transportability (31,32). Local recalibration, particularly of the smearing factor and potentially the intercept, may be required before routine deployment elsewhere. Second, the most extreme levels of proteinuria were relatively uncommon; proportional underestimation at the very high end means clinicians should interpret predictions conservatively in profoundly edematous, hemodynamically tenuous, or thrombosis-suspect presentations. Third, the prespecified thresholds (100 and 150 mg/m²/h) and the resulting ‘rule-out/gray zone/rule-in’ logic are clinically reasoned, but not yet guideline-endorsed. They should be viewed as an implementation proposal rather than a definitive standard. Fourth, the primary endpoint was proteinuria itself, not downstream hard outcomes, such as thromboembolism, albumin-requiring hypovolemia, pediatric intensive care unit transfer, or readmission. Linking the model and the gray-zone rule to those outcomes will be essential to determine whether triage truly improves safety and resource use. Finally, this model is intended for children with preserved renal function (eGFR, ≥60 ml/min/1.73 m²); performance in renal impairment requires dedicated validation. Although the equation uses spot UPCR as input, it was validated against measured 24-h protein excretion in children able to complete timed collections, and transportability to outpatient settings or to children unable to complete 24-h urine remains to be established. Future studies are thus required to focus on multicenter validation, recalibration in other populations and outcome-linked impact evaluation. In conclusion, if confirmed, this approach could reduce the burden of timed urine collection in hospitalized children, while preserving attention for those at greatest risk.