What labs should be ordered for the evaluation of hyponatremia?

Laboratory Evaluation for Hyponatremia

Initial laboratory evaluation of patients presenting with hyponatremia should include complete blood count, urinalysis, serum electrolytes (including calcium and magnesium), blood urea nitrogen, serum creatinine, glucose, fasting lipid profile, liver function tests, thyroid-stimulating hormone, serum and urine osmolality, and urine electrolytes. 1, 2

Initial Diagnostic Workup

• Confirm true hyponatremia by measuring serum osmolality to rule out pseudohyponatremia (normal serum osmolality: 275-290 mOsm/kg) 2, 3
• Measure serum and urine osmolality to determine if water excretion is normal or impaired 3, 4
• Measure urine sodium concentration to help distinguish between different causes of hyponatremia 2, 3
• Assess extracellular fluid volume status to categorize hyponatremia as hypovolemic, euvolemic, or hypervolemic 2, 3
• Measure serum uric acid level (low uric acid <4 mg/dL has a positive predictive value of 73-100% for SIADH) 2, 3

Laboratory Tests Based on Volume Status

For Hypovolemic Hyponatremia

• Urine sodium concentration (<20 mmol/L suggests hypovolemia, >40 mmol/L suggests renal sodium loss) 3, 4
• Urine osmolality (typically >100 mOsm/kg) 4
• Serum creatinine and blood urea nitrogen (often elevated) 1, 2

For Euvolemic Hyponatremia (SIADH)

• Urine sodium typically >40 mmol/L 3, 4
• Urine osmolality inappropriately elevated (>100 mOsm/kg, often >500 mOsm/kg) 3, 4
• Serum uric acid (typically <4 mg/dL) 2, 3
• Rule out hypothyroidism with thyroid-stimulating hormone 1, 2
• Rule out adrenal insufficiency with morning cortisol level 2, 3

For Hypervolemic Hyponatremia

• Urine sodium typically <20 mmol/L (except in renal failure) 3, 5
• Liver function tests to assess for cirrhosis 1, 2
• Brain natriuretic peptide (BNP) to assess for heart failure 1

Additional Tests for Specific Clinical Scenarios

• For suspected cerebral salt wasting: differentiate from SIADH by assessing volume status and fractional excretion of uric acid 2, 3
• For suspected medication-induced hyponatremia: detailed medication review 2
• For suspected endocrine disorders: cortisol level, thyroid function tests 1, 2
• For suspected liver disease: comprehensive liver function tests, including albumin 1, 5

Monitoring During Treatment

• Serial monitoring of serum sodium levels is essential during correction 1, 2
• For severe symptoms: check serum sodium every 2-4 hours during initial correction 2
• For chronic hyponatremia: check serum sodium at least daily during correction 2
• Monitor for signs of osmotic demyelination syndrome (dysarthria, dysphagia, altered mental status, quadriparesis) 2, 5

Common Pitfalls to Avoid

• Relying solely on physical examination to determine volume status (sensitivity only 41.1%, specificity 80%) 3
• Failing to rule out pseudohyponatremia in patients with normal or elevated serum osmolality 3, 4
• Misdiagnosing SIADH without excluding hypothyroidism, adrenal insufficiency, and other causes 2, 3
• Neglecting to measure both serum and urine osmolality 2, 4
• Failing to recognize cerebral salt wasting in neurosurgical patients, which requires different treatment than SIADH 2, 3

Remember that the diagnostic approach should be systematic and thorough to identify the underlying cause of hyponatremia, as this will guide appropriate treatment and prevent complications 1, 6.