What labs should be drawn to test for Syndrome of Inappropriate Antidiuretic Hormone (SIADH)?

Laboratory Tests for SIADH Diagnosis

To diagnose Syndrome of Inappropriate Antidiuretic Hormone (SIADH), the following laboratory tests should be drawn: serum sodium, serum osmolality, urine osmolality, urine sodium, serum uric acid, and assessment of volume status. 1

Essential Laboratory Tests

• Serum sodium (hyponatremia with levels <135 mmol/L is the hallmark finding) 1, 2
• Serum osmolality (will be low in SIADH) 1, 3
• Urine osmolality (typically >100 mOsm/kg and often >300 mOsm/kg, inappropriately concentrated relative to serum osmolality) 1, 2
• Urine sodium concentration (typically >30 mmol/L in SIADH; values >50 mmol/L have the highest diagnostic accuracy with 89% sensitivity and 69% specificity) 1, 4
• Serum uric acid (typically low <4 mg/dL in SIADH, with positive predictive value of 73-100%) 1, 2
• Blood urea nitrogen (BUN) (typically low in SIADH) 1, 2

Additional Tests to Rule Out Other Causes

• Serum creatinine and electrolytes (including potassium, calcium, and magnesium) 5
• Thyroid-stimulating hormone (TSH) to rule out hypothyroidism 5
• Morning cortisol or ACTH stimulation test to rule out adrenal insufficiency 1, 3
• Assessment of extracellular fluid volume status (to distinguish between SIADH and cerebral salt wasting) 5, 1

Volume Status Assessment

• Central venous pressure (CVP) measurement can help differentiate SIADH (normal volume, CVP 6-10 cm H₂O) from cerebral salt wasting (hypovolemia, CVP <6 cm H₂O) 5, 1
• Clinical assessment of volume status (though physical examination alone has been shown to be inaccurate) 5, 1

Diagnostic Criteria for SIADH

To establish a diagnosis of SIADH, the following criteria should be met:

• Hypotonic hyponatremia (serum sodium <135 mmol/L with low serum osmolality) 1, 3
• Urine osmolality inappropriately concentrated relative to serum osmolality (typically >100 mOsm/kg) 1, 3
• Urinary sodium excretion >30 mmol/L with normal salt and water intake 1, 2
• Absence of clinical evidence of volume depletion or edema 1, 3
• Normal renal, adrenal, and thyroid function 1, 3

Common Pitfalls to Avoid

• Failing to check urine osmolality and urine sodium, which are crucial for differentiating SIADH from other causes of hyponatremia 1, 2
• Relying solely on physical examination to determine volume status, which can be inaccurate 5, 1
• Not excluding other causes of hyponatremia such as hypothyroidism, adrenal insufficiency, and diuretic use 1, 6
• Confusing SIADH with cerebral salt wasting, especially in neurosurgical patients 5, 1
• Not considering that urine sodium may be low in SIADH patients with poor nutritional intake 2

Special Considerations

• In patients with lung cancer, testing for paraneoplastic syndromes including SIADH should be considered, as SIADH occurs more frequently than Cushing syndrome in these patients 5
• In patients with subarachnoid hemorrhage or other neurosurgical conditions, differentiation between SIADH and cerebral salt wasting is critical as treatment approaches differ significantly 5, 1
• Patients with high urine osmolality (>600 mOsm/kg) may be good candidates for vasopressin receptor antagonists, while those with lower urine osmolality might benefit more from fluid restriction 2