Primary Lab Abnormalities Associated with SIADH
The primary laboratory abnormalities in Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) include hyponatremia, inappropriately concentrated urine relative to plasma osmolality, elevated urine sodium, and low serum uric acid levels. 1, 2
Diagnostic Laboratory Criteria
- Hyponatremia: Serum sodium <135 mmol/L, often <130 mmol/L in clinically significant cases 1, 2
- Hypoosmolality: Low plasma osmolality (<280 mOsm/kg) 3, 4
- Inappropriately concentrated urine: Urine osmolality >100 mOsm/kg (typically >500 mOsm/kg) despite low plasma osmolality 2, 3
- Elevated urine sodium: Typically >20-30 mEq/L (often >40 mEq/L) in the setting of normal salt intake 1, 3
- Normal volume status: Laboratory findings consistent with euvolemia rather than hypovolemia or hypervolemia 1, 2
Additional Laboratory Findings
- Low serum uric acid: Levels <4 mg/dL have a positive predictive value of 73-100% for SIADH 1, 2, 5
- Normal renal function: Normal blood urea nitrogen and creatinine 6, 4
- Normal adrenal and thyroid function: Normal cortisol and thyroid-stimulating hormone levels 1, 4
- Low blood urea nitrogen: Typically lower than expected for age 5
- Lower anion gap: With nearly normal total CO₂ and serum potassium despite dilution 5
Distinguishing SIADH from Other Causes of Hyponatremia
- Volume status assessment: Central venous pressure measurements can help differentiate between SIADH (CVP 6-10 cm H₂O) and cerebral salt wasting (CVP <6 cm H₂O) 2
- Fractional excretion of sodium: Often elevated (>0.5%) in 70% of SIADH cases 5
- Fractional excretion of urea: Typically decreased in heart failure but normal or increased in SIADH 1
- Serum and urine osmolality ratio: In SIADH, urine osmolality is inappropriately high relative to serum osmolality 2, 3
Clinical Correlation with Laboratory Findings
- Severity of symptoms correlates with both the absolute serum sodium concentration and its rate of fall, particularly if greater than 0.5 mmol/L/hour 4
- Symptoms typically occur when serum sodium is ≤125 mEq/L and may include anorexia, nausea, vomiting, confusion, lethargy, seizures, and coma 2, 7
- Laboratory abnormalities may precede clinical symptoms, making regular monitoring essential 1, 4
Common Pitfalls in Laboratory Diagnosis
- Failure to exclude other causes of hyponatremia (hypothyroidism, adrenal insufficiency, diuretic use) 1, 4
- Misinterpreting low urine sodium in SIADH patients with poor nutritional intake 5
- Not recognizing that urine osmolality may not be markedly elevated in chronic SIADH or reset osmostat variant 5
- Overlooking the need to assess volume status clinically, as laboratory tests alone cannot definitively establish euvolemia 2
- Confusing SIADH with cerebral salt wasting, which presents with similar laboratory findings but with hypovolemia rather than euvolemia 1, 2