What blood and urine tests are used to diagnose and manage hyponatremia?

Blood and Urine Tests for Hyponatremia Diagnosis and Management

The initial workup for hyponatremia should include serum and urine osmolality, urine electrolytes, uric acid, and assessment of extracellular fluid volume status to determine the underlying cause. 1, 2

Initial Diagnostic Tests

• Serum sodium level (hyponatremia defined as <135 mmol/L) 1, 3
• Serum osmolality to differentiate between hypotonic, isotonic, and hypertonic hyponatremia 1, 4
• Urine osmolality to determine if water excretion is normal or impaired 1, 2
• Urine sodium concentration to help differentiate between renal and extrarenal causes 1, 5
• Serum uric acid (level <4 mg/dL has 73-100% positive predictive value for SIADH) 1, 2

Assessment of Volume Status

• Physical examination alone has limited accuracy for determining volume status (sensitivity 41.1%, specificity 80%) 2, 5
• A urinary sodium level <30 mmol/L has a positive predictive value of 71-100% for response to 0.9% saline infusion, indicating hypovolemic hyponatremia 1, 5
• In neurosurgical patients, central venous pressure (CVP) measurement may help determine intravascular volume status 2

Diagnostic Algorithm Based on Laboratory Results

Step 1: Determine Serum Osmolality

• Normal or high serum osmolality (≥275 mOsm/kg): Consider pseudohyponatremia or hyperglycemia 1, 4
• Low serum osmolality (<275 mOsm/kg): True hyponatremia, proceed to next step 2, 4

Step 2: Assess Urine Osmolality

• Urine osmolality <100 mOsm/kg: Indicates complete suppression of ADH (primary polydipsia) 1, 4
• Urine osmolality >100 mOsm/kg: Indicates impaired water excretion, proceed to next step 1, 2

Step 3: Measure Urine Sodium

• Urine sodium <20 mmol/L with hypovolemia: Extrarenal losses (GI losses, burns) 1, 4
• Urine sodium >20-40 mmol/L with hypovolemia: Renal losses (diuretics, cerebral salt wasting) 1, 5
• Urine sodium >20-40 mmol/L with euvolemia: SIADH 1, 2
• Urine sodium >20 mmol/L with hypervolemia: Advanced renal failure 1, 2

Additional Tests for Specific Causes

• Thyroid-stimulating hormone (TSH) to rule out hypothyroidism 1, 4
• Morning cortisol level to rule out adrenal insufficiency 1, 4
• Liver function tests to assess for cirrhosis 1
• Brain natriuretic peptide (BNP) to assess for heart failure 1
• Complete blood count and basic metabolic panel 1, 3

Monitoring During Treatment

• Serial serum sodium levels to ensure appropriate correction rate 1
• For severe symptoms: Monitor serum sodium every 2 hours during initial correction 1
• For chronic hyponatremia: Monitor daily to ensure correction does not exceed 8 mmol/L in 24 hours 1, 6

Common Pitfalls to Avoid

• Relying solely on physical examination to determine volume status due to poor sensitivity 2, 5
• Misdiagnosing cerebral salt wasting (CSW) as SIADH, which could lead to inappropriate fluid restriction 1, 5
• Failing to recognize that both SIADH and CSW can present with elevated urinary sodium 2, 5
• Inadequate monitoring during active correction of hyponatremia 1
• Overly rapid correction of chronic hyponatremia leading to osmotic demyelination syndrome 1, 6

Special Considerations for Neurosurgical Patients

• In neurosurgical patients, cerebral salt wasting (CSW) should be distinguished from SIADH as treatment approaches differ significantly 6, 1
• CSW is characterized by hypovolemia with elevated urinary sodium, while SIADH typically has normal volume with elevated urinary sodium 2, 5
• Vasospasm should not be treated with fluid restriction in neurosurgical patients 6