Urine Sodium in SIADH
Typical Urine Sodium Levels
In SIADH, urine sodium is characteristically elevated, typically >20-40 mEq/L, and often exceeds 40 mEq/L despite the presence of hyponatremia and hypo-osmolality. 1, 2, 3
Diagnostic Criteria and Interpretation
The diagnosis of SIADH requires several key laboratory findings that distinguish it from other causes of hyponatremia 2, 3:
- Urine sodium concentration is usually >40 mEq/L in the setting of euvolemia 3
- Urine sodium >20-40 mEq/L combined with urine osmolality >300 mOsm/kg strongly supports SIADH in euvolemic patients 1, 4
- Inappropriately elevated urine osmolality (>100 mOsm/L, typically >500 mOsm/kg) despite low serum osmolality (<275 mOsm/kg) 2, 3, 4
- Serum osmolality <275 mOsm/kg with serum sodium <134-135 mEq/L 2, 3
Pathophysiology of Elevated Urine Sodium
The elevated urine sodium in SIADH occurs through a specific mechanism 2:
- Inappropriate ADH secretion causes water retention and dilutional hyponatremia 2, 3
- The body maintains fluid balance through physiologic natriuresis - sodium is excreted to prevent volume overload 2
- This natriuresis occurs at the expense of plasma sodium, resulting in the paradox of high urine sodium despite hyponatremia 2
- Fluid balance is preserved while plasma sodium falls 2
Clinical Significance of Very High Urine Sodium
Extremely high urine sodium concentrations (>130 mmol/L) can occur in severe SIADH and may predict poor response to fluid restriction alone 5:
- Very high urine osmolality (>500 mOsm/kg) combined with persistently elevated urine sodium suggests more severe SIADH 5
- These patients often require more aggressive treatment beyond simple fluid restriction 5
- Fluid administration in these cases paradoxically worsens hyponatremia 5
Differential Diagnosis Considerations
A urine sodium <30 mmol/L has 71-100% positive predictive value for hypovolemic hyponatremia and good response to saline infusion, effectively ruling out SIADH 1:
- Urine sodium <30 mmol/L suggests extrarenal sodium losses (GI losses, dehydration) 1
- Urine sodium >20 mmol/L with clinical euvolemia points toward SIADH 1, 2
- In neurosurgical patients, distinguish SIADH (euvolemic, CVP 6-10 cm H₂O) from cerebral salt wasting (hypovolemic, CVP <6 cm H₂O, both have high urine sodium >20 mmol/L) 1, 2
Treatment Implications Based on Urine Studies
Initial urine osmolality <400 mOsm/kgH₂O predicts that approximately 30% of chronic SIADH patients can be successfully treated with moderate water restriction (1.5-2 L/day) alone 6:
- Patients with urine osmolality >400 mOsm/kgH₂O typically require additional therapy such as urea (15-30g daily) 6
- Very high baseline urine osmolality (>595 mOsm/kgH₂O) often necessitates higher urea doses (30g daily) combined with fluid restriction 6
- Urine osmolality increases during successful treatment in most patients 6
Common Pitfalls
Do not use normal saline (0.9% NaCl) to treat SIADH - it acts as a hypotonic solution relative to the concentrated urine and can paradoxically worsen hyponatremia through rapid fluctuations in serum sodium 4:
- Normal saline provides sodium that gets excreted in concentrated urine while free water is retained 4
- This can cause rapid correction during infusion (risking osmotic demyelination) followed by post-infusion worsening 4
- Fluid restriction to 1 L/day is the cornerstone of SIADH treatment, not saline administration 1, 2, 4