SIADH Diagnostic Criteria
The diagnosis of SIADH requires five essential criteria: hypotonic hyponatremia (serum sodium <135 mmol/L), inappropriately concentrated urine (urine osmolality >100 mOsm/kg, typically >500 mOsm/kg), elevated urinary sodium (>20 mEq/L), euvolemic state (absence of clinical signs of hypovolemia or hypervolemia), and normal renal, adrenal, and thyroid function. 1, 2
Essential Diagnostic Criteria
Laboratory Requirements:
- Serum sodium <135 mmol/L with plasma osmolality <275 mOsm/kg 1, 3
- Urine osmolality inappropriately elevated (>100 mOsm/kg) relative to low plasma osmolality, typically >500 mOsm/kg 1, 2, 4
- Urine sodium concentration >20 mEq/L (often >40 mEq/L), indicating natriuresis despite hyponatremia 1, 5, 4, 6
Clinical Assessment:
- Euvolemic state - no clinical evidence of volume depletion (orthostatic hypotension, dry mucous membranes, decreased skin turgor) or volume overload (peripheral edema, ascites, jugular venous distention) 1, 2, 7
- Normal renal function - adequate kidney function to exclude other causes 1, 2
- Normal adrenal and thyroid function - must exclude hypothyroidism and adrenal insufficiency before confirming SIADH 1, 2, 6
Supporting Laboratory Findings
Additional markers that support SIADH diagnosis:
- Serum uric acid <4 mg/dL has a positive predictive value of 73-100% for SIADH 8, 7
- Low blood urea nitrogen - typically low due to dilution 4
- Fractional excretion of sodium >0.5% in approximately 70% of cases 4
- Lower anion gap with nearly normal total CO2 and serum potassium despite dilution 4
Critical Differential Diagnosis Considerations
Volume status assessment is paramount to distinguish SIADH from mimics:
- Cerebral salt wasting (CSW) presents with hypovolemia (CVP <6 cm H₂O), orthostatic hypotension, and unquenchable thirst, requiring volume replacement rather than fluid restriction 7, 8
- SIADH presents with euvolemia (CVP 6-10 cm H₂O) and no signs of volume depletion 7
- Hypovolemic hyponatremia shows urine sodium <30 mmol/L (in contrast to SIADH's >20 mEq/L) 8, 6
Exclude these conditions before diagnosing SIADH:
- Thiazide diuretic use - screen all patients 6
- Hypothyroidism - check TSH 8, 6
- Adrenal insufficiency - assess cortisol function 8, 6
- Pseudohyponatremia - measure plasma osmolality to exclude hyperglycemia, hyperlipidemia, or hyperproteinemia 3
- Reset osmostat - urine osmolality will be appropriately low (<100 mOsm/kg) at lower sodium set points 4
- Primary polydipsia - urine osmolality <100 mOsm/kg with excessive fluid intake 7, 4
Common Diagnostic Pitfalls
Failing to assess volume status accurately is the most common error, as physical examination alone has poor sensitivity (41.1%) and specificity (80%) 8
Using normal saline in confirmed SIADH can paradoxically worsen hyponatremia, as the sodium load triggers further natriuresis while the free water is retained 6
Obtaining ADH levels is not supported by evidence and should not delay diagnosis or treatment 8
Distinguishing SIADH from CSW in neurosurgical patients is critical, as CSW is more common than SIADH in this population and requires fundamentally opposite treatment (volume replacement vs. fluid restriction) 8, 1