Diagnostic Criteria for SIADH
SIADH is diagnosed when hypotonic hyponatremia (serum Na <131 mmol/L) occurs with inappropriately concentrated urine (urine osmolality >100 mOsm/kg), clinical euvolemia, urine sodium >30 mmol/L, and after excluding other causes of euvolemic hyponatremia such as hypothyroidism, hypocortisolism, and polydipsia. 1
Essential Diagnostic Criteria
The diagnosis requires meeting all of the following mandatory criteria 1, 2:
- Hypotonic hyponatremia: Serum sodium <131 mmol/L with serum osmolality <275 mOsm/kg
- Inappropriately concentrated urine: Urine osmolality >100 mOsm/kg (typically exceeds plasma osmolality)
- Clinical euvolemia: Normal extracellular fluid (ECF) volume status
- Natriuresis: Urine sodium concentration >30 mmol/L
- Normal renal, adrenal, and thyroid function
- Absence of volume depletion or edema
Algorithmic Diagnostic Approach
Step 1: Confirm True Hyponatremia
When serum Na <131 mmol/L, measure serum and urine osmolality 1. Normal or high serum osmolarity suggests pseudohyponatremia from hyperglycemia or hypertriglyceridemia rather than true SIADH.
Step 2: Assess Volume Status
This is the critical distinguishing step between SIADH and cerebral salt wasting (CSW) 1. Physical examination alone is inadequate (sensitivity only 41%, specificity 80%) 1. Use:
Central venous pressure (CVP) when available:
- CVP 6-10 cm H₂O = normovolemic (consistent with SIADH)
- CVP <6 cm H₂O = hypovolemic (suggests CSW)
- CVP >10 cm H₂O = hypervolemic (suggests heart failure, cirrhosis, renal failure)
Clinical assessment (less reliable): Look for absence of orthostatic hypotension, normal skin turgor, no jugular venous distention, no peripheral edema
Step 3: Laboratory Confirmation
Obtain the following supportive tests 1, 3:
- Urine sodium >30 mmol/L (present in most SIADH cases)
- Fractional excretion of sodium >0.5% (seen in 70% of SIADH)
- Serum uric acid <4 mg/dL (positive predictive value 73-100% for SIADH) 1
- Low blood urea nitrogen (typical in SIADH)
- Normal or near-normal total CO₂ and potassium despite dilution 3
Step 4: Exclude Other Causes
Before confirming SIADH, rule out 1:
- Hypothyroidism
- Adrenal insufficiency (hypocortisolism)
- Polydipsia
- Diuretic use
- Cirrhosis, congestive heart failure, renal failure (if hypervolemic)
Supporting Criteria (Need ≥3 Additional)
Beyond the mandatory criteria, the diagnosis is strengthened by meeting at least 3 of these 1:
- Abnormal water load test
- Elevated plasma AVP levels relative to plasma osmolality
- No significant correction with volume expansion
- Correction with fluid restriction
Critical Pitfalls to Avoid
Do not measure ADH or natriuretic peptide levels 1. The literature does not support their use (Class III evidence), as ADH can be detectable in all hyponatremic patients and "appropriateness" of ADH levels is poorly defined 1.
Do not rely on physical examination alone to determine volume status 1. The sensitivity is too low (41%) and can lead to misdiagnosis of CSW as SIADH, resulting in dangerous fluid restriction when volume expansion is needed.
Distinguish SIADH from CSW carefully in neurosurgical patients 1. Both present with hyponatremia and natriuresis, but CSW requires volume replacement while SIADH requires fluid restriction. Misdiagnosis can lead to cerebral infarction in CSW patients treated with fluid restriction 1.
Clinical Context
A serum sodium ≤131 mmol/L warrants full evaluation and treatment 1. Symptoms correlate with both the absolute sodium level and rate of decline—particularly dangerous when falling >0.5 mmol/L/hour 1. Seizures typically occur when sodium drops below 120-121 mmol/L 1.
The diagnosis is ultimately one of exclusion after systematic evaluation combining physical examination findings, basic laboratory studies, and invasive monitoring when available 1.