Diagnostic Criteria for SIADH
SIADH is diagnosed by demonstrating hypotonic hyponatremia (serum sodium <134-135 mEq/L) with inappropriately concentrated urine (osmolality >500 mosm/kg), elevated urinary sodium (>20-40 mEq/L), and clinical euvolemia, after excluding hypothyroidism, adrenal insufficiency, and volume depletion. 1, 2
Essential Diagnostic Criteria
The diagnosis requires meeting all five cardinal criteria simultaneously:
- Hypotonic hyponatremia: Serum sodium <134-135 mEq/L with plasma osmolality <275 mosm/kg 1, 2, 3
- Inappropriately concentrated urine: Urine osmolality >500 mosm/kg (or at minimum >100 mosm/kg) despite low plasma osmolality 1, 2, 3
- Elevated urinary sodium: Urine sodium concentration >20-40 mEq/L, reflecting physiologic natriuresis in response to volume expansion 1, 2, 4
- Clinical euvolemia: Absence of edema, orthostatic hypotension, dry mucous membranes, jugular venous distention, or ascites 1, 3
- Normal renal, thyroid, and adrenal function: Must exclude hypothyroidism, adrenal insufficiency, and renal failure as alternative causes 1, 2, 3
Critical Volume Status Assessment
Determining euvolemia is the most crucial step in diagnosing SIADH, as it distinguishes SIADH from hypovolemic and hypervolemic causes of hyponatremia. 1, 5
Physical examination findings for euvolemia include:
- No peripheral edema, ascites, or jugular venous distention (excludes hypervolemia) 1
- No orthostatic hypotension, dry mucous membranes, or decreased skin turgor (excludes hypovolemia) 1
- Moist mucous membranes and normal skin turgor 2
However, physical examination alone has poor accuracy (sensitivity 41.1%, specificity 80%) for determining volume status, so laboratory parameters are essential. 1, 5
Supportive Laboratory Findings
Additional laboratory findings that support the diagnosis:
- Low serum uric acid (<4 mg/dL) has a positive predictive value of 73-100% for SIADH 1, 5, 2
- Low blood urea nitrogen (BUN), reflecting dilution and increased urea clearance 4
- Fractional excretion of sodium >0.5% in approximately 70% of cases 4
- Normal or near-normal serum potassium and total CO2 despite dilution 4
- Lower anion gap compared to other causes of hyponatremia 4
Special Diagnostic Considerations in Neurosurgical Patients
In patients with CNS pathology (particularly subarachnoid hemorrhage), distinguishing SIADH from cerebral salt wasting (CSW) is critical because they require opposite treatments. 1, 5, 2
Key distinguishing features:
| Feature | SIADH | Cerebral Salt Wasting |
|---|---|---|
| Volume status | Euvolemic | Hypovolemic |
| Central venous pressure | 6-10 cm H₂O | <6 cm H₂O [2] |
| Clinical signs | No dehydration | Orthostatic hypotension, tachycardia, dry mucous membranes [2] |
| Treatment | Fluid restriction | Volume and sodium replacement [1,2] |
Common Diagnostic Pitfalls
- Failing to assess volume status accurately is the most common error, leading to misdiagnosis and inappropriate treatment 1, 2
- Relying solely on physical examination without laboratory confirmation of euvolemia 1, 5
- Obtaining ADH and natriuretic peptide levels is not supported by evidence and should not be done 1, 5
- Ignoring mild hyponatremia (130-135 mmol/L) as clinically insignificant, when it actually increases fall risk and mortality 5
- Misdiagnosing CSW as SIADH in neurosurgical patients, leading to harmful fluid restriction instead of needed volume replacement 1, 2
Diagnostic Algorithm
- Confirm true hypotonic hyponatremia: Check serum osmolality to exclude pseudohyponatremia from hyperglycemia or hyperlipidemia 1
- Assess volume status: Clinical examination plus laboratory parameters (uric acid, BUN, fractional excretion of sodium) 1, 4
- Check urine studies: Urine osmolality >500 mosm/kg and urine sodium >20-40 mEq/L 1, 2
- Exclude other causes: TSH for hypothyroidism, morning cortisol for adrenal insufficiency, serum creatinine for renal failure 1, 2
- In neurosurgical patients: Consider central venous pressure measurement if available to distinguish SIADH from CSW 2