From the Guidelines
For a SIADH workup in the emergency department, order serum sodium, serum osmolality, urine osmolality, urine sodium, and serum uric acid as the core diagnostic tests. These labs are essential in establishing the diagnosis of SIADH, which is characterized by euvolemic hypoosmolar hyponatremia, inappropriately high urine osmolality, and inappropriately high urinary sodium concentration 1. Begin with these labs along with a basic metabolic panel, complete blood count, thyroid function tests, and cortisol level to establish the diagnosis and rule out other causes of hyponatremia. Some key findings to look for 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 Additional helpful tests include serum glucose to rule out hyperglycemia-induced hyponatremia, and kidney and liver function tests to assess for other organ dysfunction. Consider ordering chest imaging and additional studies based on suspected underlying causes, as SIADH is often secondary to conditions like malignancy, CNS disorders, or medication effects. Volume status assessment is crucial to differentiate SIADH from other hyponatremic states, as SIADH patients are typically euvolemic 1.
From the Research
Laboratory Workup for SIADH in the Emergency Department
To diagnose and manage Syndrome of Inappropriate Antidiuretic Hormone (SIADH) in the emergency department, several laboratory tests are essential. The following labs should be ordered as part of the SIADH workup:
- Serum sodium level: This is crucial for diagnosing hyponatremia, a key feature of SIADH 2.
- Serum osmolality: Helps to differentiate between hypotonic and isotonic hyponatremia 3.
- Urine sodium: Elevated urine sodium (>30 mEq/L) is characteristic of SIADH, reflecting inappropriate antidiuretic hormone secretion 3.
- Urine osmolality: High urine osmolality (>600 mOsm/kg) indicates that the kidneys are inappropriately concentrating urine in the presence of hyponatremia, consistent with SIADH 3.
- Fractional excretion of sodium (FeNa): Can help distinguish between SIADH and other causes of hyponatremia, with most SIADH patients having a FeNa >0.5% 3.
- Urea and uric acid levels: Low levels of these can be seen in SIADH, although they are less specific, especially in elderly patients 3.
- Renal function tests (e.g., creatinine, eGFR): To assess for any underlying renal impairment that could affect sodium and water handling 4.
- Thyroid function tests and cortisol level: To rule out hypothyroidism and adrenal insufficiency, which can also cause hyponatremia 2.