What are the tests for Syndrome of Inappropriate Antidiuretic Hormone (SIADH)?

From the Guidelines

SIADH is diagnosed through a combination of clinical assessment and laboratory tests, including hyponatremia, hypoosmolality, inappropriately high urine osmolality, and inappropriately high urinary sodium concentration.

Diagnostic Criteria

The diagnostic criteria for SIADH include:

• Hyponatremia (serum sodium < 134 mEq/L) 1
• Hypoosmolality (plasma osmolality < 275 mosm/kg) 1
• Inappropriately high urine osmolality (> 500 mosm/kg) 1
• Inappropriately high urinary sodium concentration (> 20 mEq/L) 1
• Absence of hypothyroidism or adrenal insufficiency or volume depletion 1

Laboratory Tests

Laboratory findings in SIADH include:

• Urine osmolality of > 300 mosm/kg 1
• Urinary sodium level of > 40 mEq/L 1
• Serum osmolality of < 275 mosm/kg 1
• Serum uric acid concentration of < 4 mg/dL 1

Clinical Assessment

Clinical assessment of intravascular volume status and biochemical measurements in blood and urine are also essential in diagnosing SIADH 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. After exclusion of hypoadrenalism and hypothyroidism, hypoosmolar hyponatremia in the absence of renal salt wasting, dehydration, and intravascular volume depletion will be consistent with SIADH 1.

From the Research

Diagnosis of SIADH

The diagnosis of SIADH should be considered if the five cardinal criteria are fulfilled, including:

• Hypotonic hyponatraemia
• Natriuresis
• Urine osmolality in excess of plasma osmolality
• Absence of oedema and volume depletion
• Normal renal and adrenal function 2 Other diagnostic factors include:
• Low levels of uric acid, typically seen in 70% of SIADH patients 3
• Low urea levels, although this is less specific for elderly patients 3
• High urine sodium (>30 mEq/L) and fractional excretion of Na (>0.5% in 70% of cases) 3
• Measurement of urine osmolality to diagnose polydipsia and reset osmostat 3

Laboratory Tests

Laboratory tests are necessary for the diagnosis of SIADH, including:

• Serum sodium levels
• Urine osmolality
• Urine sodium
• Urea and uric acid levels
• Plasma osmolality 3, 4 In severe, symptomatic cases of hyponatremia, patients may need treatment before the results of laboratory tests can be obtained 4

Assessment of Patients

When diagnosing a patient with hyponatremia, the first steps should include:

• Assessing the severity of neurological symptoms
• Checking the patient's volemic status to determine whether emergency treatment with hypertonic saline is indicated 4 A high index of suspicion for patients at risk is critical to prompt recognition of SIADH 5