What lab tests are used to diagnose and manage Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)?

Laboratory Tests for Diagnosing and Managing SIADH

The diagnosis of SIADH requires a comprehensive panel of laboratory tests including serum and urine osmolality, urine sodium, and exclusion of other causes of hyponatremia through specific biochemical measurements. 1, 2

Essential Diagnostic Laboratory Tests

• Serum sodium measurement (hyponatremia defined as <135 mmol/L) is the initial test that prompts further investigation 1, 2
• Serum osmolality (typically <275 mOsm/kg in SIADH) to confirm hypotonic hyponatremia and rule out pseudohyponatremia 1, 2
• Urine osmolality (typically >100 mOsm/kg and often >500 mOsm/kg in SIADH) to demonstrate inappropriate urinary concentration 1, 3
• Urine sodium concentration (typically >20-40 mEq/L in SIADH) to help differentiate causes of hyponatremia 1, 2
• Serum uric acid (typically <4 mg/dL in SIADH with a positive predictive value of 73-100%) 1, 2, 4
• Blood urea nitrogen (BUN) (typically low in SIADH) 1, 4

Exclusionary Tests to Rule Out Other Causes

• Thyroid function tests (TSH) to exclude hypothyroidism 1, 2
• Morning cortisol or ACTH stimulation test to exclude adrenal insufficiency 1, 2
• Liver function tests to assess for liver disease 1
• Kidney function tests (creatinine, estimated GFR) to exclude renal failure 1, 2
• Complete blood count to assess for other underlying conditions 1
• Glucose levels to rule out hyperglycemia as a cause of hyponatremia 1

Tests to Assess Volume Status

• Clinical assessment of extracellular fluid volume status (though physical examination alone has limited accuracy with sensitivity of only 41.1% and specificity of 80%) 2, 5
• Central venous pressure (CVP) measurements can help differentiate between SIADH (CVP 6-10 cm H₂O) and cerebral salt wasting (CVP <6 cm H₂O) 3
• Fractional excretion of sodium (FENa) (though this has limitations in the elderly, with 42% of SIADH patients having FENa <0.5%) 5
• Fractional excretion of urea (may be helpful but has limitations with only 52% of SIADH patients having FE urea >50%) 5

Monitoring Tests During Treatment

• Serial serum sodium measurements are essential during correction of hyponatremia 1
  • Every 2-4 hours during initial correction for severe symptoms 1
  • Daily during fluid restriction or other treatments 1
• Fluid balance monitoring 1
• Serum potassium to monitor for hypokalemia during treatment 1

Special Considerations

• In neurosurgical patients, additional testing may be needed to differentiate SIADH from cerebral salt wasting, as treatment approaches differ significantly 1, 2, 3
• The saline infusion test (2 liters of isotonic saline over 24 hours) may help differentiate SIADH from salt depletion, but must consider both sodium level changes and sodium excretion 5
• Plasma renin and aldosterone measurements have poor discriminative value in elderly patients 5

Common Pitfalls to Avoid

• Relying solely on physical examination to determine volume status (low sensitivity) 2
• Failing to measure both serum and urine osmolality simultaneously 1, 6
• Not excluding other causes of hyponatremia with similar laboratory findings (e.g., hypothyroidism, adrenal insufficiency) 1, 2
• Misinterpreting urine sodium levels in patients on diuretics 1, 6
• Overlooking the need for serial monitoring of serum sodium during correction to prevent osmotic demyelination syndrome 1