What laboratory tests are used to diagnose Hyponatremia due to Syndrome of Inappropriate Antidiuretic Hormone (SIADH) secretion?

From the Guidelines

To diagnose hyponatremia due to SIADH, laboratory tests such as serum sodium, serum osmolality, urine sodium, and urine osmolality are crucial, with serum sodium typically being low (< 134 mEq/L), serum osmolality decreased (< 275 mosm/kg), urine sodium elevated (> 20 mEq/L), and urine osmolality inappropriately high (> 500 mosm/kg) 1. The diagnosis of SIADH involves a combination of clinical assessment and laboratory findings. Key laboratory tests include:

• Serum sodium to confirm hyponatremia
• Serum osmolality to assess hypoosmolality
• Urine sodium to evaluate inappropriate sodium excretion
• Urine osmolality to assess the kidney's ability to concentrate urine Additional tests to rule out other causes of hyponatremia and to assess the patient's volume status and organ function include:
• Thyroid function tests (TSH, free T4) to exclude hypothyroidism
• Morning cortisol to rule out adrenal insufficiency
• Serum creatinine and BUN to evaluate kidney function
• Measurement of serum glucose to rule out hyperglycemia as a cause of pseudohyponatremia
• A comprehensive metabolic panel to assess liver function and rule out other electrolyte abnormalities These tests help distinguish SIADH from other causes of hyponatremia by demonstrating euvolemia, normal renal, adrenal, and thyroid function, along with the characteristic pattern of inappropriate water retention and concentrated urine despite low serum sodium 1. The clinical assessment of intravascular volume status and biochemical measurements in blood and urine can also aid in the diagnosis, with the fractional excretion of urate helping to assess the effective arterial blood volume and increase the accuracy of the diagnostic algorithm for SIADH 1.

From the Research

Laboratory Tests for Diagnosing Hyponatremia due to SIADH

The following laboratory tests are used to diagnose hyponatremia due to Syndrome of Inappropriate Antidiuretic Hormone (SIADH) secretion:

• Serum sodium and osmolality levels to confirm hypotonic hyponatremia 2, 3, 4, 5
• Urea and uric acid levels, with low levels of uric acid being more specific for SIADH 2
• Anion gap and total CO2 levels to differentiate SIADH from other causes of hyponatremia 2
• Serum potassium levels, which are typically normal in SIADH 2
• Urine sodium and osmolality levels, with high urine sodium (>30 mEq/L) and osmolality (>100 mosmol/hgH2O) being characteristic of SIADH 2, 6, 3, 5
• Fractional excretion of sodium, which can help differentiate SIADH from other causes of hyponatremia 2

Urine Biology in SIADH Diagnosis

Urine biology can be helpful in diagnosing SIADH, with the following characteristics:

• High urine sodium concentrations (>30 mEq/L) 2, 6, 3, 5
• High urine osmolality (>500 mOsm/kg) 6, 3
• Fractional excretion of sodium >0.5% in 70% of cases 2

Diagnostic Criteria for SIADH

The diagnosis of SIADH is made based on the following criteria:

• Hypotonic hyponatremia 4, 5
• Natriuresis 4
• Urine osmolality in excess of plasma osmolality 4
• Absence of edema and volume depletion 4
• Normal renal and adrenal function 4