What are the laboratory findings for the diagnosis of Syndrome of Inappropriate Antidiuretic Hormarmone (SIADH)?

From the Guidelines

The laboratory diagnosis of SIADH requires several key findings, including hyponatremia (serum sodium <135 mEq/L) with decreased serum osmolality (<280 mOsm/kg), inappropriately concentrated urine osmolality (>100 mOsm/kg), and elevated urine sodium (>20 mEq/L) 1. To diagnose SIADH, the following steps should be taken:

• Check for hyponatremia with decreased serum osmolality
• Assess urine osmolality and sodium levels
• Evaluate volume status to ensure euvolemia
• Rule out other causes of euvolemic hyponatremia, such as hypothyroidism, adrenal insufficiency, and renal dysfunction, by measuring thyroid function tests, cortisol levels, and kidney function tests 1
• Review medications, as many can cause SIADH
• Consider additional tests, such as measuring ADH levels or conducting a water loading test, if available It is essential to note that the provided evidence does not directly address the diagnosis of SIADH, but rather focuses on heart failure and adrenal insufficiency. However, the principles of diagnosing and managing hyponatremia and euvolemic disorders can be applied to SIADH diagnosis. The pathophysiology of SIADH involves excessive ADH release or action, leading to increased water reabsorption in the kidneys, diluting serum sodium while concentrating urine despite the body's reduced need for water conservation. In clinical practice, it is crucial to prioritize the diagnosis and management of SIADH to prevent complications such as cerebral edema and seizures. By following these steps and considering the underlying pathophysiology, clinicians can accurately diagnose and manage SIADH, improving patient outcomes and quality of life.

From the Research

Lab Diagnosis of SIADH

To diagnose the Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH), several laboratory tests and clinical criteria are considered. The key features include:

• Hyponatremia (low serum sodium) 2, 3, 4, 5, 6
• Low serum osmolality 2, 3, 4, 5
• High urine sodium (>30 mEq/L) 3, 5, 6
• High urine osmolality (>100 mOsm/kg) 4, 5
• Absence of edema and volume depletion 2, 3
• Normal renal and adrenal function 3, 4

Diagnostic Criteria

The diagnosis of SIADH is based on the following criteria:

• Hypotonic hyponatremia
• Natriuresis (high urine sodium)
• Urine osmolality in excess of plasma osmolality
• Absence of edema and volume depletion
• Normal renal and adrenal function 3

Laboratory Tests

Laboratory tests that can help in the diagnosis of SIADH include:

• Serum sodium and osmolality
• Urine sodium and osmolality
• Urea and uric acid levels (typically low in SIADH) 5
• Fractional excretion of sodium (>0.5% in 70% of cases) 5
• Anion gap and total CO2 levels (usually normal in SIADH) 5

Differential Diagnosis

The differential diagnosis of SIADH includes other causes of hyponatremia, such as:

• Hypovolemic hyponatremia
• Hypocritic hyponatremia
• Polydipsic hyponatremia
• Reset osmostat 3, 5, 6

Urine Sodium Value

The urine sodium value can be useful in differentiating SIADH from hypovolemic hyponatremia, with a value of 50 mEq/L being the most accurate in separating the two conditions 6.