Diagnostic Accuracy of Urinary Potassium Index Versus Inferior Vena Cava Collapsibility Index for Detection of Hypovolaemia in Oedematous Children with Idiopathic Nephrotic Syndrome

Background Accurate assessment of intravascular volume status in children with idiopathic nephrotic syndrome (INS) presenting with oedema is essential before initiating diuretics or albumin. The urinary potassium index (UKI) has been proposed as a non-invasive surrogate for renin-angiotensin-aldosterone system (RAAS) activation; however, its diagnostic accuracy against a rigorous composite reference standard has not been established. Our objective was to evaluate the diagnostic performance of UKI for hypovolaemia in oedematous children with INS. Materials and Methods This prospective observational study enrolled 80 children aged 1–12 years with INS in relapse (cases) and 59 controls (31 nephrotic remission, 28 healthy) at a tertiary paediatric nephrology unit (October 2022–December 2023). Hypovolaemia was defined by ≥2 of 3 domains: clinical, biochemical (haematocrit >40%, BUr/SCr ratio >100, or serum aldosterone >250 pg/mL), or radiological (IVCCI >50%). UKI and IVCCI were compared against this composite standard using ROC curve analysis. Results Hypovolaemia was present in 30 of 80 (37.5%) cases. UKI was higher in cases (0.57 ± 0.28) than in nephrotic controls (0.36 ± 0.24; p = 0.001) and healthy controls (0.42 ± 0.15; p = 0.018), but did not discriminate between hypovolaemic and non-hypovolaemic cases (AUC 0.610; 95% CI 0.484–0.737; p = 0.100). IVCCI demonstrated excellent accuracy (AUC 0.956; 95% CI 0.901–1.000; sensitivity 93.3%, specificity 90.0%; p <0.001). UKI correlated positively with serum aldosterone (r = +0.421, p <0.001) and negatively with FENa (r = −0.813, p <0.001) and serum albumin (r = −0.345, p = 0.002). Conclusions UKI is not a reliable standalone marker for hypovolaemia in oedematous children with INS. IVCCI demonstrated superior diagnostic utility. A composite multiparameter approach remains essential for safe volume-directed management.