Diagnostic Utility of Fractional Excretion of Urate, Urinary Phosphate and Brain Natriuretic Peptide in Distinguishing Cerebral/Renal Salt Wasting From SIADH in Neurologically Ill Children: A Systematic Review

ABSTRACT Background Hyponatremia remains a common and life‐threatening complication in children with traumatic brain injury, subarachnoid haemorrhage, brain tumours and post neurosurgical states, yet distinguishing its two primary neurogenic etiologies—syndrome of inappropriate antidiuretic hormone secretion (SIADH) and cerebral/renal salt wasting (C/RSW)—remains difficult. Misdiagnosis may result in inappropriate treatment strategies with a significant risk of morbidity. While adult studies suggest diagnostic roles for fractional excretion of uric acid (FEurate), fractional excretion of phosphate (PEphosphate) and NT‐proBNP level, no paediatric‐specific guidelines or reference thresholds currently exist. Objective To evaluate the diagnostic accuracy of FEurate, FEphosphate and NT‐proBNP in distinguishing C/RSW from SIADH in hyponatremic children with neurologic conditions. Data Sources A comprehensive literature search was performed across PubMed, Embase, Web of Science, Scopus and the Cochrane Library. Study Selection and Data Extraction Eligible studies included RCTs, systematic reviews, meta‐analysis and prospective/retrospective studies involving paediatric patients diagnosed with SIADH or C/RSW using any of the three biomarkers. Systematic reviews were appraised using AMSTAR 2. Two reviewers independently assessed eligibility and extracted data. Results Out of 69 identified studies, there were only two prospective paediatric cohort studies reported on the use of biomarkers in C/RSW versus SIADH syndromes. Limitation Findings are limited by the absence of paediatric‐specific cut‐off values. Conclusions Current evidence suggests that FEurate, FEphosphate and NT‐proBNP may provide diagnostic clues, but each biomarker has limitations related to assay variability, age‐dependent reference ranges, comorbidities and confounding medications. FEurate response to sodium correction appears most useful, though serial measurements are often required.