How to Diagnose SIADH
SIADH is diagnosed by demonstrating hypotonic hyponatremia (serum sodium <135 mEq/L, plasma osmolality <275 mosm/kg) with inappropriately concentrated urine (urine osmolality >500 mosm/kg) and elevated urinary sodium (>20 mEq/L) in a euvolemic patient, after excluding hypothyroidism, adrenal insufficiency, and volume depletion. 1
Essential Diagnostic Criteria
The diagnosis requires all five cardinal criteria to be met 2:
- Hypotonic hyponatremia: Serum sodium <134-135 mEq/L with plasma osmolality <275 mosm/kg 1, 2
- Inappropriately concentrated urine: Urine osmolality >500 mosm/kg relative to low plasma osmolality 1, 2
- Elevated urinary sodium: Urine sodium concentration >20-40 mEq/L despite hyponatremia 1, 3
- Euvolemic state: Absence of clinical signs of hypovolemia (orthostatic hypotension, dry mucous membranes, decreased skin turgor) or hypervolemia (edema, ascites, jugular venous distention) 4, 5
- Normal renal, adrenal, and thyroid function: Must exclude hypothyroidism and adrenal insufficiency as alternative causes 1, 2
Critical Volume Status Assessment
Accurately determining euvolemia is the most critical step in distinguishing SIADH from other causes of hyponatremia. 4, 5
Clinical Signs of Euvolemia in SIADH
- No peripheral edema 5
- No orthostatic hypotension 5
- Normal skin turgor 5
- Moist mucous membranes 5
- Absence of jugular venous distention 4
Distinguishing SIADH from Cerebral Salt Wasting (CSW)
This distinction is particularly important in neurosurgical patients 4, 1:
- SIADH: Euvolemic with CVP 6-10 cm H₂O 5
- CSW: Hypovolemic with CVP <6 cm H₂O, evidence of volume depletion (hypotension, tachycardia), and unquenchable thirst 5
Laboratory Workup
Initial Tests Required
- Serum sodium and osmolality: Confirm hyponatremia and hypoosmolality 4
- Urine osmolality and sodium: Demonstrate inappropriate urinary concentration (>500 mosm/kg) and sodium excretion (>20 mEq/L) 1, 3
- Serum creatinine and BUN: Assess renal function 4
- Thyroid-stimulating hormone (TSH): Rule out hypothyroidism 4
- Cortisol level: Exclude adrenal insufficiency 4
Supportive Laboratory Findings
- Serum uric acid <4 mg/dL: Has 73-100% positive predictive value for SIADH, though may also occur in CSW 4, 1
- Low BUN: Typically <10 mg/dL due to dilution 4
Tests NOT Supported by Evidence
- Measuring plasma ADH levels is not supported by evidence and should not delay treatment 4
- Natriuretic peptide levels are not useful for diagnosis 4
Common Diagnostic Pitfalls
- Failing to assess volume status accurately: Physical examination alone has poor sensitivity (41.1%) and specificity (80%) 4
- Misdiagnosing CSW as SIADH in neurosurgical patients: CSW is more common than SIADH in patients with subarachnoid hemorrhage and requires fundamentally different treatment (volume replacement vs. fluid restriction) 4, 1
- Not excluding medication-induced SIADH: Common culprits include SSRIs, carbamazepine, chlorpropamide, cyclophosphamide, vincristine, cisplatin, and NSAIDs 1, 6
- Overlooking underlying malignancy: SIADH affects 1-5% of lung cancer patients, particularly small cell lung cancer 4, 1
Underlying Causes to Investigate
Once SIADH is diagnosed, identify the underlying cause 2:
- Malignancy: Especially small cell lung cancer, but also other thoracic and abdominal tumors 1
- CNS disorders: Meningitis, encephalitis, head trauma, stroke, subarachnoid hemorrhage 2
- Pulmonary diseases: Pneumonia, tuberculosis, positive pressure ventilation 2
- Medications: Review all current medications for known SIADH-inducing agents 1, 6
- Post-operative state: Particularly with hypotonic fluid administration 2