What are the criteria for diagnosing Hyponatremia (low sodium levels in the blood) due to Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH)?

From the Guidelines

Hyponatremia due to Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is diagnosed when a patient presents with euvolemic hypoosmolar hyponatremia, characterized by serum sodium < 134 mEq/L, 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 1. The diagnosis of SIADH involves a combination of clinical assessment and laboratory findings. Key criteria include:

• Hyponatremia (serum sodium < 134 mEq/L)
• Hypoosmolality (plasma osmolality < 275 mosm/kg)
• Inappropriately high urine osmolality (> 500 mosm/kg)
• Inappropriately high urinary sodium concentration (> 20 mEq/L)
• Absence of hypothyroidism or adrenal insufficiency or volume depletion These criteria help distinguish SIADH from other causes of hyponatremia, such as hypovolemia, heart failure, cirrhosis, hypothyroidism, and adrenal insufficiency 1. The pathophysiology of SIADH involves excessive ADH release or action, leading to increased water reabsorption in the kidneys, dilutional hyponatremia, and the characteristic laboratory findings of concentrated urine despite low serum osmolality 1. Proper diagnosis requires systematic exclusion of other causes and careful evaluation of all criteria, including clinical assessment of intravascular volume status and biochemical measurements in blood and urine 1. By assessing the effective arterial blood volume with the fractional excretion of urate, the accuracy of a diagnostic algorithm for SIADH can approach 95% 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.

From the Research

Diagnostic Criteria for SIADH

The diagnosis of Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH) is based on the following criteria:

• Clinical euvolemic state, meaning no signs of dehydration or fluid overload 2, 3
• Low serum sodium levels, indicating hyponatremia 4, 2, 3, 5, 6
• Low serum osmolality, indicating a dilution of sodium in the blood 2, 3
• Elevated urine sodium levels, indicating excessive sodium excretion in the urine 2, 3
• Elevated urine osmolality, indicating concentrated urine 3
• Exclusion of pseudohyponatremia and diuretic use, which can cause similar laboratory results 2, 3

Laboratory Measurements

Laboratory measurements are crucial in diagnosing SIADH, including:

• Serum sodium levels, to confirm hyponatremia 4, 2, 3, 5, 6
• Serum osmolality, to assess the concentration of solutes in the blood 2, 3
• Urine sodium levels, to evaluate sodium excretion in the urine 2, 3
• Urine osmolality, to assess the concentration of solutes in the urine 3

Clinical Considerations

Clinical considerations are also important in diagnosing SIADH, including:

• Screening for thiazide use and adrenal or thyroid dysfunction, which can cause similar symptoms 3
• Ruling out cerebral salt wasting and reset osmostat, which can mimic SIADH 3
• Assessing the severity of hyponatremia and the presence of symptoms, such as altered mental state or seizures 2, 3, 5, 6