Is a urine sodium level of 170 mEq/L considered low in the context of hyponatremia?

A Urine Sodium of 170 mEq/L is NOT Low—It is Markedly Elevated

A urine sodium of 170 mEq/L is extremely high, not low, and indicates significant renal sodium wasting in the context of hyponatremia. This level is more than triple the threshold typically used to distinguish between different causes of hyponatremia 1.

Understanding Urine Sodium Thresholds in Hyponatremia

Normal Diagnostic Cutoffs

• Urine sodium <30 mmol/L suggests extrarenal sodium losses or appropriate renal sodium conservation (hypovolemic hyponatremia from non-renal causes) 2, 1
• Urine sodium >20-40 mEq/L indicates renal sodium wasting or inappropriate urinary sodium excretion 1, 3
• Urine sodium <50-70 mEq/L before or after loop diuretics may reflect heightened kidney sodium avidity in heart failure 2

Your Value of 170 mEq/L

This is profoundly elevated and indicates one of the following conditions:

• SIADH (Syndrome of Inappropriate Antidiuretic Hormone): Characterized by inappropriately elevated urinary sodium (>20-40 mEq/L) despite hyponatremia, with urine osmolality typically >300-500 mOsm/kg 1, 3
• Cerebral Salt Wasting (CSW): Particularly in neurosurgical patients, with urine sodium >20 mEq/L despite volume depletion 4, 1
• Diuretic use: Loop or thiazide diuretics cause obligatory renal sodium losses 2, 5
• Adrenal insufficiency: Results in renal sodium wasting 1
• Salt-losing nephropathy: Primary renal tubular disorders 1

Clinical Interpretation Algorithm

Step 1: Assess volume status 1, 6

• Hypovolemic (orthostatic hypotension, dry mucous membranes, decreased skin turgor) → Consider CSW, diuretic use, or adrenal insufficiency
• Euvolemic (no edema, normal blood pressure) → SIADH is most likely
• Hypervolemic (edema, ascites, jugular venous distention) → Advanced renal failure or heart failure with diuretic use

Step 2: Check urine osmolality 1, 6

• >300-500 mOsm/kg with high urine sodium → SIADH or CSW
• <100 mOsm/kg → Primary polydipsia (but urine sodium would typically be low)

Step 3: Additional tests to distinguish SIADH from CSW 4, 1

• Serum uric acid <4 mg/dL: 73-100% positive predictive value for SIADH 4, 1
• Central venous pressure: CVP <6 cm H₂O suggests CSW; CVP 6-10 cm H₂O suggests SIADH 1
• Clinical context: Recent neurosurgery, subarachnoid hemorrhage → CSW more likely 4

Common Pitfall to Avoid

Do not confuse high urine sodium with low urine sodium. A urine sodium of 170 mEq/L represents massive renal sodium losses—the kidneys are inappropriately excreting sodium when they should be conserving it in the setting of hyponatremia 2, 1. This is the opposite of sodium avidity, where urine sodium would be <30 mEq/L 2.

In cirrhotic patients with ascites, inadequate urinary sodium excretion (<50-70 mEq/L) after loop diuretics reflects heightened kidney sodium avidity and poor diuretic response 2. Your value of 170 mEq/L would indicate the opposite—excessive sodium excretion.

Treatment Implications Based on Urine Sodium of 170 mEq/L

If SIADH (euvolemic): 4, 1, 3

• Fluid restriction to 1 L/day as first-line treatment
• Avoid normal saline—it will worsen hyponatremia
• Consider oral sodium chloride 100 mEq three times daily if fluid restriction fails
• Vaptans (tolvaptan 15 mg daily) for resistant cases

If CSW (hypovolemic): 4, 1

• Volume and sodium replacement with isotonic or hypertonic saline
• Never use fluid restriction—this worsens outcomes
• Consider fludrocortisone 0.1-0.2 mg daily for severe cases
• Aggressive volume resuscitation is critical

Correction rate limits regardless of cause: 4, 1, 3

• Maximum 8 mmol/L increase in 24 hours to prevent osmotic demyelination syndrome
• High-risk patients (cirrhosis, alcoholism, malnutrition): 4-6 mmol/L per day