Urine sodium levels post-saline infusion in differentiating non-edematous hyponatremia

Accurately distinguishing hypovolemic hyponatremia from the syndrome of inappropriate antidiuresis (SIAD) remains challenging due to limitations in traditional pre-saline infusion urine sodium measurements. This prospective cohort study at two Thai medical centers evaluated whether post-saline infusion urine sodium improves diagnostic accuracy. We enrolled 113 hospitalized adults with non-edematous hypotonic hyponatremia (serum sodium < 130 mmol/L). Asymptomatic patients received 500 mL of 0.9% saline, while symptomatic patients received 150 mL of 3% saline. Urine sodium was measured before and after infusion (Time 1). Diagnostic performance was assessed using AUC curves, with Youden's J statistic determining optimal cutoffs. Post-infusion urine sodium at Time 1 demonstrated superior accuracy compared to pre-infusion measurements (AUC 0.75 vs. 0.61, P = 0.01). A cutoff of 24.5 mmol/L at Time 1 achieved 75.2% accuracy (95% CI: 66.2-82.9), 62.5% sensitivity (95% CI: 45.8-77.3), and 82.2% specificity (95% CI: 71.5-90.2). These findings highlight the clinical utility of post-saline urine sodium. In conclusion, measuring urine sodium after saline infusion significantly improves differentiation between hypovolemic hyponatremia and SIAD compared to pre-infusion testing. The 24.5 mmol/L cutoff at Time 1 offers a practical, evidence-based threshold for clinicians, reducing misdiagnosis risks and guiding appropriate fluid management in hyponatremic patients.