SIADH Workup
When serum sodium drops below 131 mmol/L, immediately obtain serum and urine osmolality, urine sodium, urine electrolytes, and serum uric acid, while simultaneously assessing extracellular fluid volume status through physical examination findings and central venous pressure monitoring when available. 1
Initial Laboratory Assessment
The diagnostic workup must include:
- Serum osmolality - to confirm hypotonic hyponatremia (normal or high osmolality suggests pseudohyponatremia from hyperglycemia or hypertriglyceridemia)
- Urine osmolality - inappropriately elevated (>100 mOsm/kg) in SIADH despite low serum osmolality
- Urine sodium - typically >30 mmol/L in SIADH
- Serum uric acid - levels <4 mg/dL have 73-100% positive predictive value for SIADH 1
- Urine electrolytes including fractional excretion of sodium and urea 1
Critical Volume Status Determination
Physical examination alone is inadequate for determining volume status (sensitivity only 41.1%, specificity 80%) 1. The workup must include:
Physical Examination Parameters
- Mucosal hydration
- Skin turgor
- Jugular vein distention
- Orthostatic vital signs (pulse increase >10%, systolic BP decrease >10% when upright)
- Presence/absence of edema 1
Invasive Monitoring When Available
Central venous pressure (CVP) is the most reliable method to distinguish SIADH from cerebral salt wasting:
- CVP 6-10 cm H₂O = normovolemic (SIADH)
- CVP <5-6 cm H₂O = hypovolemic (cerebral salt wasting)
- CVP >10 cm H₂O = hypervolemic 1
Algorithmic Approach to Diagnosis
Step 1: Confirm Hypotonic Hyponatremia
If serum osmolality is normal/high → investigate pseudohyponatremia, not SIADH
Step 2: Categorize by Volume Status
Hypovolemic (CVP <5 cm H₂O):
- Urine Na <30 mmol/L → extrarenal losses
- Urine Na >30 mmol/L → cerebral salt wasting, diuretics, or adrenal insufficiency
Normovolemic (CVP 6-10 cm H₂O):
- Rule out thyroid disease, hypocortisolism, and polydypsia before diagnosing SIADH 1
Hypervolemic (CVP >10 cm H₂O):
- Consider cirrhosis, congestive heart failure, renal failure (less common in neurosurgical patients) 1
Step 3: Etiologic Investigation for Confirmed SIADH
At minimum, obtain:
- Chest radiograph or CT scan - to evaluate for malignancy and pulmonary disease 2
- Medication review - numerous drugs cause SIADH 3, 4
- Neurological assessment - CNS pathology is a major cause 3
The four main etiologic categories are:
Important Caveats
Do NOT routinely measure ADH or natriuretic peptide levels - these have limited diagnostic value and are not supported by evidence. SIADH can occur with undetectable ADH levels, and the "appropriateness" of ADH levels is not well-defined 1.
Urinary sodium <30 mmol/L has 71-100% positive predictive value that isotonic saline will increase serum sodium - this helps identify saline-responsive hypovolemia rather than true SIADH 1.
In elderly patients, SIADH is often idiopathic (60% of cases) with the main identifiable causes being pneumonia (18%) and medications (12%). Extensive diagnostic workup beyond history, physical exam, medication review, and chest x-ray is often not warranted in this population 4.
The distinction between acute (<48 hours) versus chronic (>48 hours) hyponatremia is critical for treatment planning and must be determined during initial workup, as this affects correction rates and risk of osmotic demyelination syndrome 5.