Diagnostic Criteria for SIADH
The diagnosis of SIADH requires hypotonic hyponatremia (serum sodium <135 mmol/L, plasma osmolality <275 mOsm/kg) with inappropriately concentrated urine (urine osmolality >100 mOsm/kg, typically >500 mOsm/kg) and elevated urinary sodium (>20 mEq/L, typically >40 mEq/L) in a clinically euvolemic patient with normal thyroid, adrenal, and renal function. 1, 2, 3
Essential Diagnostic Criteria
The five cardinal criteria that must be fulfilled for SIADH diagnosis are: 2
- Hypotonic hyponatremia - Serum sodium <135 mmol/L with plasma osmolality <275 mOsm/kg 1, 4
- Inappropriately concentrated urine - Urine osmolality >100 mOsm/kg (typically >500 mOsm/kg) despite low plasma osmolality 1, 2, 3
- Elevated urinary sodium concentration - Urine sodium >20 mEq/L (typically >40 mEq/L) indicating natriuresis 1, 2, 3
- Clinical euvolemia - Absence of edema, orthostatic hypotension, normal skin turgor, and moist mucous membranes 1, 2
- Normal renal, adrenal, and thyroid function - Exclusion of other causes of hyponatremia 1, 2, 3
Clinical Assessment of Volume Status
Accurate assessment of extracellular fluid volume status is critical to differentiate SIADH from other causes of hyponatremia, particularly cerebral salt wasting which presents with hypovolemia rather than euvolemia. 5, 6
Euvolemia in SIADH is characterized by: 1
- No peripheral edema
- No orthostatic hypotension
- Normal skin turgor
- Moist mucous membranes
- Absence of jugular venous distention
- No signs of dehydration or volume overload
Laboratory Workup
The initial diagnostic evaluation should include: 5
- Serum studies: Sodium, osmolality, creatinine, glucose, thyroid-stimulating hormone (TSH), and cortisol to rule out hypothyroidism and adrenal insufficiency 5, 1
- Urine studies: Osmolality, sodium concentration, and spot urine sodium 5, 1
- Serum uric acid: A level <4 mg/dL has a positive predictive value of 73-100% for SIADH 5, 1
Common Diagnostic Pitfalls
Failing to accurately assess volume status is the most common pitfall in SIADH diagnosis, as this is essential for differentiating it from cerebral salt wasting (hypovolemic) and heart failure/cirrhosis (hypervolemic). 1, 5
Additional pitfalls to avoid: 5, 1
- Obtaining ADH and natriuretic peptide levels is not supported by evidence and should not delay treatment 5
- Misdiagnosing volume status in patients with underlying conditions like heart failure or cirrhosis 5
- Failing to exclude pseudohyponatremia from hyperglycemia, hyperlipidemia, or hyperproteinemia 4
- Not recognizing that SIADH remains a diagnosis of exclusion after ruling out other causes 3
Special Considerations
In neurosurgical patients, particularly those with subarachnoid hemorrhage, distinguishing SIADH from cerebral salt wasting is critical as they require fundamentally different treatments—SIADH requires fluid restriction while cerebral salt wasting requires volume and sodium replacement. 5, 1 Central venous pressure can help differentiate: SIADH typically shows CVP 6-10 cm H₂O (euvolemic) versus cerebral salt wasting with CVP <6 cm H₂O (hypovolemic). 1