From the Guidelines
The first lab test to order when suspecting Syndrome of Inappropriate Antidiuretic Hormone (SIADH) secretion is serum sodium level, along with serum and urine osmolality. When evaluating a patient with possible SIADH, checking serum sodium is essential as hyponatremia (low sodium in the blood, typically below 135 mEq/L) is the hallmark finding of this condition 1. This initial test should be accompanied by simultaneous measurement of serum osmolality, which will be low in SIADH (typically less than 275 mOsm/kg), and urine osmolality, which will be inappropriately high relative to the serum osmolality (typically greater than 100 mOsm/kg and often above 300 mOsm/kg) 1. These tests together provide the foundation for diagnosis, as SIADH is characterized by hyponatremia with inappropriately concentrated urine despite low serum osmolality. This pattern occurs because excess ADH causes the kidneys to retain water inappropriately, diluting the sodium in the bloodstream while producing concentrated urine even when the body should be excreting dilute urine to correct the hyponatremia.
Key points to consider when ordering these lab tests include:
- Hyponatremia is a critical finding, and its severity can guide the urgency of treatment 1
- Serum osmolality and urine osmolality are crucial for differentiating SIADH from other causes of hyponatremia, such as adrenal insufficiency or hypothyroidism 1
- The clinical context, including the presence of euvolemia and the absence of other causes of hyponatremia, is essential for making a diagnosis of SIADH 1
By prioritizing the measurement of serum sodium, serum osmolality, and urine osmolality, clinicians can efficiently diagnose SIADH and initiate appropriate management to prevent complications associated with severe hyponatremia, such as seizures, coma, and death 1.
From the Research
Initial Lab Tests for Suspected SIADH
When suspecting Syndrome of Inappropriate Antidiuretic Hormone (SIADH) secretion, the first lab tests to consider include:
- Serum sodium levels to confirm hyponatremia 2, 3, 4, 5, 6
- Serum osmolality to assess hypo-osmolality 2, 3, 6
- Urine sodium concentration to evaluate inappropriate sodium excretion 2, 3, 4, 6
- Urine osmolality to assess the kidney's ability to concentrate or dilute urine 2, 3
Key Lab Findings in SIADH
Key lab findings that support the diagnosis of SIADH include:
- Hyponatremia with corresponding hypo-osmolality of the serum and extracellular fluid 2, 3, 6
- Elevated urine sodium concentration (>30-40 mEq/L) 2, 3, 4
- Urine osmolality greater than that appropriate for the concomitant osmolality of the plasma 2, 3
- Low urea and uric acid levels 3
- Normal anion gap and total CO2 levels 3
Diagnostic Approach
The diagnostic approach to SIADH involves a combination of clinical evaluation and laboratory tests to exclude other causes of hyponatremia and to confirm the presence of inappropriate ADH secretion 2, 3, 6. The urine sodium concentration can be a useful initial test to differentiate SIADH from hypovolemic hyponatremia 4.