Diagnostic Criteria for SIADH
SIADH diagnosis requires hypotonic hyponatremia with inappropriately concentrated urine in a normovolemic patient, after excluding thyroid disease, adrenal insufficiency, and polydipsia. 1
Essential Diagnostic Criteria
The diagnosis of SIADH must meet all of the following mandatory criteria 1, 2:
- Hypotonic hyponatremia: Serum sodium <131 mmol/L with low serum osmolality 1
- Inappropriately concentrated urine: Urine osmolality exceeding plasma osmolality despite hypotonicity 2, 3
- Natriuresis: Urine sodium typically >30 mmol/L (reflecting continued salt intake) 3, 4
- Normovolemia: Absence of clinical volume depletion or edema 2, 5
- Normal renal and adrenal function: Must exclude renal failure and adrenal insufficiency 2
Additional Supporting Criteria
At least 3 of the following should be present to strengthen the diagnosis 1:
- Low serum uric acid: <4 mg/dL (positive predictive value 73-100% for SIADH) 1
- Low blood urea nitrogen: Typically reduced due to dilution 4
- Fractional excretion of sodium >0.5%: Present in approximately 70% of SIADH cases 4
- Normal or near-normal total CO2 and serum potassium: Despite dilution 4
- Lower anion gap: Characteristic finding in SIADH 4
Critical Exclusions Required Before Diagnosis
The following conditions MUST be ruled out before confirming SIADH 1:
- Thyroid disease: Hypothyroidism can mimic SIADH 1
- Hypocortisolism/adrenal insufficiency: Check cortisol levels; these patients typically have lower total CO2 than non-endocrine SIADH 1, 4
- Polydipsia: Urine osmolality measurement helps distinguish this 4
- Hypervolemic states: Cirrhosis, congestive heart failure, renal failure 1
- Hypovolemia/cerebral salt wasting: Particularly important in neurosurgical patients 1
- Diuretic use: Must be excluded as cause of natriuresis 1
Practical Diagnostic Approach
Initial workup for serum sodium <131 mmol/L should include 1:
- Serum and urine osmolality
- Urine sodium and electrolytes
- Serum uric acid
- Assessment of extracellular fluid (ECF) volume status
- Thyroid and adrenal function tests
Volume Status Assessment
ECF volume status is the key distinguishing feature between SIADH and cerebral salt wasting 1. Physical examination alone has poor sensitivity (41.1%) and specificity (80%) for determining volume status 1. When available, central venous pressure (CVP) provides more reliable assessment: CVP 6-10 cm H₂O suggests normovolemia consistent with SIADH, while CVP <6 cm H₂O indicates hypovolemia suggesting cerebral salt wasting 1.
Important Caveats
ADH levels are NOT recommended for diagnosis, as they have limited diagnostic value 1. SIADH has been documented even with undetectable ADH levels, and the "appropriateness" of ADH levels remains undefined 1. Similarly, natriuretic peptide levels are not supported by evidence for SIADH diagnosis 1.
Urine sodium <30 mmol/L does not exclude SIADH in patients with poor oral intake or salt restriction 4. Conversely, high urine sodium in the setting of adequate salt intake is expected in SIADH 4.
Pseudohyponatremia must be excluded: Normal or high serum osmolarity may result from laboratory error, hyperglycemia, or hypertriglyceridemia 1.