What are the initial lab tests for a patient presenting with hyponatremia?

Initial Laboratory Tests for Hyponatremia

For a patient presenting with hyponatremia, obtain serum and urine osmolality, urine sodium concentration, serum electrolytes (including calcium and magnesium), blood urea nitrogen, serum creatinine, glucose, and thyroid-stimulating hormone as the essential initial workup. 1, 2

Core Laboratory Panel

Serum Tests (First Priority)

• Serum osmolality to differentiate hypotonic from nonhypotonic hyponatremia (normal range: 275-290 mOsm/kg) 1, 3, 4
• Serum sodium to confirm hyponatremia (<135 mmol/L) and assess severity 1, 3
• Serum electrolytes including potassium, calcium, and magnesium 5, 1
• Blood urea nitrogen (BUN) and serum creatinine to assess renal function and volume status 5, 1, 4
• Fasting blood glucose (or glycohemoglobin) to exclude hyperglycemia-induced pseudohyponatremia 5, 1
• Thyroid-stimulating hormone (TSH) to rule out hypothyroidism 5, 1, 4

Urine Tests (Essential for Diagnosis)

• Urine osmolality to determine if water excretion is impaired (>100 mOsm/kg suggests impaired free water excretion) 1, 2, 3, 4
• Urine sodium concentration to differentiate causes:
  • <30 mmol/L suggests hypovolemic hyponatremia with extrarenal losses 1, 2

  • 20-40 mmol/L suggests SIADH, cerebral salt wasting, or renal losses 1, 2, 3

Additional Helpful Tests

• Serum uric acid - levels <4 mg/dL have 73-100% positive predictive value for SIADH 1, 2
• Complete blood count as part of comprehensive evaluation 5, 1
• Liver function tests to assess for cirrhosis in hypervolemic states 5, 1
• Urinalysis for general assessment 5, 1

Diagnostic Algorithm Based on Laboratory Results

Step 1: Confirm True Hyponatremia

• Measure serum osmolality first to exclude pseudohyponatremia from hyperglycemia, hypertriglyceridemia, or laboratory error 3, 4
• Calculate corrected sodium: add 1.6 mEq/L to measured sodium for each 100 mg/dL glucose >100 mg/dL 1

Step 2: Assess Urine Osmolality

• <100 mOsm/kg: Appropriate ADH suppression (primary polydipsia, beer potomania) 1, 4
• >100 mOsm/kg: Impaired water excretion, proceed to urine sodium 1, 4

Step 3: Measure Urine Sodium

• <30 mmol/L: Suggests hypovolemia from extrarenal losses (GI losses, dehydration) 1, 2, 4
• >20-40 mmol/L: Suggests SIADH (if euvolemic), cerebral salt wasting (if hypovolemic), or renal losses 1, 2, 4

Step 4: Clinical Volume Assessment

• Hypovolemic signs: Orthostatic hypotension, dry mucous membranes, decreased skin turgor 1, 2
• Euvolemic: No edema, normal blood pressure, normal skin turgor 1, 2
• Hypervolemic signs: Peripheral edema, ascites, jugular venous distention 1, 2

Tests NOT Recommended

• ADH levels - obtaining ADH is not supported by evidence and should not delay treatment 1, 2
• Natriuretic peptide levels for hyponatremia diagnosis - not supported by literature 1, 2

Common Pitfalls to Avoid

• Relying solely on physical examination for volume status has poor accuracy (sensitivity 41.1%, specificity 80%) 1, 2
• Failing to check serum osmolality first can miss pseudohyponatremia from hyperglycemia 3, 4
• Not obtaining urine studies prevents differentiation between SIADH and cerebral salt wasting, which require opposite treatments 1, 2
• Ignoring TSH and cortisol can miss secondary causes requiring specific treatment 6, 4

Special Considerations

Neurosurgical Patients

• Distinguish between SIADH and cerebral salt wasting by assessing central venous pressure (CVP <6 cm H₂O suggests cerebral salt wasting) 1, 2
• Both conditions show elevated urine sodium, but volume status differs critically 1, 2

Cirrhotic Patients

• Check liver function tests and albumin as hyponatremia reflects worsening hemodynamic status 1
• Sodium ≤130 mEq/L increases risk for hepatic encephalopathy, hepatorenal syndrome, and spontaneous bacterial peritonitis 1