Plasma Sodium Levels in SIADH
Patients with SIADH typically present with hyponatremia, defined as serum sodium <135 mEq/L, with most clinically significant cases falling below 130-131 mmol/L. 1, 2
Defining Hyponatremia in SIADH
- Hyponatremia is defined as serum sodium <135 mEq/L, though clinical significance and need for intervention typically begins when levels drop below 130-131 mmol/L 1, 2
- Moderate hyponatremia ranges from 120-125 mEq/L, requiring more aggressive management 1
- Severe hyponatremia is defined as serum sodium <120 mEq/L, representing a medical emergency when symptomatic 1, 3
Characteristic Laboratory Findings in SIADH
SIADH is characterized by a specific constellation of laboratory abnormalities that distinguish it from other causes of hyponatremia:
- Hypotonic hyponatremia with plasma osmolality <275 mOsm/kg 2, 4
- Inappropriately concentrated urine with osmolality >500 mOsm/kg despite low serum osmolality 2, 4
- Elevated urinary sodium concentration typically >20-40 mEq/L, often exceeding 40 mEq/L 2, 4
- Clinical euvolemia (absence of edema, orthostatic hypotension, or signs of volume depletion) 2, 4
- Normal renal, adrenal, and thyroid function 2, 4
Severity Classification and Clinical Implications
The degree of hyponatremia in SIADH directly correlates with morbidity and mortality:
- Mild hyponatremia (130-135 mEq/L) is associated with increased fall risk (21% vs 5% in normonatremic patients) and should not be dismissed as clinically insignificant 1
- Moderate hyponatremia (<130 mEq/L) carries a 60-fold increase in hospital mortality (11.2% vs 0.19%) compared to normonatremic patients 1
- Severe symptomatic hyponatremia (<120 mEq/L with neurological symptoms) requires immediate intervention with hypertonic saline 1, 2
Urine Studies That Confirm SIADH
Beyond the serum sodium level, specific urine findings are diagnostic:
- Urine osmolality persistently >100 mOsm/kg (typically >500 mOsm/kg) indicates impaired water excretion 2, 4
- Urine sodium concentration >20-40 mEq/L reflects physiologic natriuresis occurring to maintain fluid balance at the expense of plasma sodium 2, 4
- Very high urine sodium concentrations (>130 mmol/L) can occur in severe SIADH and predict poor response to fluid restriction 5
- Serum uric acid <4 mg/dL has a positive predictive value of 73-100% for SIADH 1
Common Pitfalls in Diagnosis
- Failing to distinguish SIADH from cerebral salt wasting (CSW) in neurosurgical patients is critical, as CSW presents with hypovolemia (CVP <6 cm H₂O) despite similar urine sodium levels, requiring opposite treatment approaches 1, 2
- Ignoring mild hyponatremia (130-135 mEq/L) as clinically insignificant overlooks the increased risk of falls, cognitive impairment, and mortality 1
- Assuming all patients with high urine sodium have SIADH without confirming euvolemia and excluding other causes like diuretic use or renal salt wasting 1