What are the laboratory criteria for Syndrome of Inappropriate Antidiuretic Hormone (SIADH) secretion?

From the Guidelines

The laboratory criteria for diagnosing Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) include hyponatremia (serum sodium < 134 mEq/L), hypoosmolality (plasma osmolality < 275 mosm/kg), inappropriately high urine osmolality (> 500 mosm/kg), and inappropriately high urinary sodium concentration (> 20 mEq/L) in the absence of diuretic treatment, adrenal insufficiency, heart failure, cirrhosis, and hypothyroidism. When evaluating patients for SIADH, it is essential to assess the degree of hyponatremia and the acuity of hypoosmolality, as signs and symptoms can vary from mild to life-threatening 1. Some key points to consider in the diagnosis of SIADH include:

• Hyponatremia should be further investigated by clinical assessment of intravascular volume status and biochemical measurements in blood and urine 1
• Laboratory findings in SIADH include urine osmolality of > 300 mosm/kg, urinary sodium level of > 40 mEq/L, serum osmolality of < 275 mosm/kg, and serum uric acid concentration of < 4 mg/dL 1
• The absence of hypothyroidism or adrenal insufficiency or volume depletion is crucial in the diagnosis of SIADH 1
• Paraneoplastic hyponatremia secondary to elevated atrial natriuretic peptide has also been described and should be distinguished from SIADH in the differential diagnosis of hyponatremia 1. The management of SIADH is based on expert opinion, and recommended treatments include free water restriction, hypertonic 3% saline IV, demeclocycline, lithium, and vasopressin 2 receptor antagonists 1.

From the Research

Lab Criteria for SIADH

The following lab criteria are used to diagnose SIADH:

• Decreased serum osmolality 2
• Inappropriately elevated urine osmolality (>100 mosmol/L) 2
• Elevated urine sodium (Na) levels 2, 3, 4
• Low urea levels, although this is less specific for elderly patients 3
• Low levels of uric acid, seen in 70% of SIADH patients 3
• Lower anion gap with nearly normal total CO2 and serum potassium 3
• High fractional excretion of Na (>0.5% in 70% of cases) 3
• Urine sodium (Na) >30 mEq/L 3, 4

Additional Diagnostic Considerations

When diagnosing SIADH, it is also important to:

• Rule out thiazide use 2
• Rule out adrenal or thyroid dysfunction 2
• Consider clinical mimics of SIADH, such as cerebral salt wasting and reset osmostat 2
• Measure urine osmolality to help choose a therapeutic strategy 3