What labs should be ordered for a patient presenting with hyponatremia (low sodium levels)?

Diagnostic Laboratory Evaluation for Hyponatremia

The initial workup for a patient presenting with hyponatremia should include serum sodium, serum osmolality, urine osmolality, urine sodium concentration, thyroid function tests, and cortisol levels to determine the underlying cause and guide appropriate treatment. 1

Initial Laboratory Assessment

• Serum measurements:

  • Serum sodium level (confirms hyponatremia if <135 mEq/L)
  • Serum osmolality (normal: 275-295 mOsm/kg)
  • Serum uric acid (<4 mg/dL suggests SIADH)
  • Glucose (to rule out pseudohyponatremia from hyperglycemia)
  • Lipid profile (to rule out pseudohyponatremia from hypertriglyceridemia)
  • Thyroid function tests (TSH, free T4)
  • Cortisol level (to rule out adrenal insufficiency)

• Urine measurements:

  • Urine osmolality (key for differentiating causes)
  • Urine sodium concentration
  • Fractional excretion of urate (improves diagnostic accuracy for SIADH to 95%) 2

Diagnostic Algorithm Based on Volume Status

Volume status assessment is crucial for determining the cause of hyponatremia 1:

1. Hypovolemic Hyponatremia

• Clinical signs: Orthostatic hypotension, tachycardia, dry mucous membranes
• Laboratory findings:
  • Urine sodium: <20 mEq/L (suggests extrarenal losses)
  • Urine sodium: >20 mEq/L (suggests renal losses - diuretics, adrenal insufficiency)
• Potential causes: Volume depletion, diuretic use, vomiting, diarrhea, cerebral salt wasting

2. Euvolemic Hyponatremia

• Laboratory findings:
  • Urine osmolality: >500 mOsm/kg
  • Urine sodium: >20-40 mEq/L
• Diagnostic criteria for SIADH (must meet all):
  • Hyponatremia (serum sodium <134 mEq/L)
  • Hypoosmolality (plasma osmolality <275 mOsm/kg)
  • Inappropriately high urine osmolality (>500 mOsm/kg)
  • Inappropriately high urinary sodium (>20 mEq/L)
  • Absence of hypothyroidism, adrenal insufficiency, or volume depletion 2

3. Hypervolemic Hyponatremia

• Clinical signs: Edema, ascites, elevated jugular venous pressure
• Laboratory findings:
  • Urine sodium: <20 mEq/L
  • Elevated urine osmolality
• Potential causes: Heart failure, cirrhosis, nephrotic syndrome, renal failure

Special Considerations

1. Rule out pseudohyponatremia:

   • Check serum osmolality - if normal or high, consider laboratory error or pseudohyponatremia from hyperglycemia or hypertriglyceridemia 2

2. Medication review:

   • Identify medications that may cause hyponatremia (SSRIs, carbamazepine, thiazide diuretics, NSAIDs) 1

3. Severity classification:

   • Mild: 130-134 mmol/L
   • Moderate: 125-129 mmol/L
   • Severe: <125 mmol/L 3

4. Chronicity assessment:

   • Acute: <48 hours
   • Chronic: >48 hours 1

Important Caveats

• Obtaining levels of hormones such as ADH and natriuretic peptides is not supported by the literature for routine diagnosis 2

• In neurosurgical patients, cerebral salt wasting should be distinguished from SIADH as management differs significantly 2

• Early detection and appropriate management can prevent severe complications including seizures, coma, and death 1

• Even mild chronic hyponatremia is associated with cognitive impairment, gait disturbances, and increased rates of falls and fractures 3

By following this systematic laboratory approach, clinicians can accurately diagnose the underlying cause of hyponatremia and implement appropriate treatment strategies to improve patient outcomes.