The Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is a complex condition concerning the body’s water balance that requires specialized testing for accurate diagnosis. This disorder results from the excessive or inappropriate release of Antidiuretic Hormone (ADH), also known as vasopressin, which is normally responsible for regulating water reabsorption in the kidneys. When ADH is released inappropriately, the kidneys hold onto too much water. This retention dilutes the blood, leading directly to hyponatremia, defined as an abnormally low concentration of sodium in the blood. Diagnosis involves interpreting a series of blood and urine measurements against strict clinical criteria.
Recognizing Symptoms That Require Testing
The symptoms that prompt testing for SIADH are primarily driven by the resulting hyponatremia. Early, milder signs are often non-specific and may include nausea, malaise, and headache. As hyponatremia becomes more severe, symptoms become neurological, such as lethargy, confusion, or difficulty concentrating. Severe presentations involve neurological dysfunction, including seizures, delirium, or coma. Clinical suspicion for SIADH is high when hyponatremia is present without obvious signs of fluid overload or significant dehydration, though mild cases are often discovered incidentally.
Essential Blood and Urine Measurements
The diagnostic process relies on a paired analysis of both blood and urine to understand water and sodium handling. The first step confirms hyponatremia, with serum sodium concentration falling below the normal range of 135 mEq/L. This low sodium level must be accompanied by a low serum osmolality (below 275 mOsm/kg), confirming hypotonic hyponatremia. These two blood tests establish the fundamental state of water excess in the body.
The next measurements analyze the urine to determine kidney response. In SIADH, the urine osmolality is inappropriately concentrated, typically measuring greater than 100 mOsm/kg. This is irregular because a healthy body with diluted blood should be producing maximally dilute urine. Finally, the urine sodium concentration is usually elevated, often greater than 40 mEq/L, reflecting the body’s attempt to excrete sodium due to volume expansion.
Confirming Diagnosis Using Clinical Criteria
Confirming a diagnosis of SIADH involves interpreting the laboratory data in the context of the patient’s overall fluid status. Simultaneous presence of hyponatremia and hypotonicity confirms a state of water excess. High urine osmolality confirms the inappropriate action of ADH, showing that the kidneys are actively concentrating the urine instead of diluting it. The high urine sodium concentration further supports the diagnosis, indicating that the kidneys are wasting salt despite the body’s low serum sodium level. This salt wasting is a consequence of the slight increase in total body water, which expands the extracellular volume.
A central requirement for the SIADH diagnosis is the patient’s clinical euvolemia, meaning they have a normal fluid volume status. Euvolemia is assessed clinically by looking for the absence of signs of fluid overload, such as peripheral edema, or signs of volume depletion, like orthostatic hypotension. The combination of hyponatremia, low serum osmolality, and inappropriately concentrated urine with high urine sodium in a euvolemic patient confirms the pathological process of inappropriate ADH secretion.
Excluding Conditions That Mimic SIADH
SIADH is formally considered a diagnosis of exclusion, meaning that several other common causes of hyponatremia must be ruled out before the diagnosis can be confirmed. A thorough differential diagnosis is mandatory to ensure the patient receives the correct treatment. Common mimics are conditions where the body is releasing ADH appropriately, such as severe volume depletion or heart failure.
To exclude hypovolemia, the patient is assessed for dehydration, and their urine sodium level is evaluated; true volume depletion results in a very low urine sodium concentration. Adrenal insufficiency must be ruled out using tests like a baseline cortisol level or an ACTH stimulation test. Hypothyroidism is excluded by checking thyroid function tests, and renal impairment is ruled out by checking serum creatinine and blood urea nitrogen levels. Only after confirming the patient is euvolemic and has normal adrenal, thyroid, and renal function can the diagnosis of SIADH be finalized.