Laboratory Findings in SIADH
In SIADH, expect hyponatremia (serum sodium <135 mmol/L) with low serum osmolality (<275 mOsm/kg), inappropriately concentrated urine (urine osmolality >100 mOsm/kg, typically >500 mOsm/kg), elevated urine sodium (>20-40 mEq/L), and clinical euvolemia without edema or signs of volume depletion. 1, 2
Serum Laboratory Findings
Serum Sodium and Osmolality:
- Hyponatremia with serum sodium <135 mmol/L (often <120 mmol/L in severe cases) 1, 3
- Low plasma osmolality (<275 mOsm/kg) is pathognomonic for SIADH 3, 2
- The severity ranges from mild (126-135 mmol/L) to moderate (120-125 mmol/L) to severe (<120 mmol/L) 4
Serum Uric Acid:
- Low serum uric acid (<4 mg/dL) has a positive predictive value of 73-100% for SIADH 4, 5
- This finding is present in approximately 70% of SIADH cases, compared to only 40% in salt-depleted patients 5
Blood Urea Nitrogen (BUN):
- Typically low BUN due to dilutional effect and increased urea clearance 5
- This is less specific in elderly patients who may have higher baseline values due to reduced urea clearance 5
Electrolyte Panel:
- Lower anion gap despite dilution 5
- Nearly normal total CO2 (bicarbonate) 5
- Serum potassium typically normal despite dilution 5
- Normal serum creatinine (excludes renal insufficiency as a cause) 6, 5
Urine Laboratory Findings
Urine Osmolality:
- Inappropriately elevated urine osmolality (>100 mOsm/kg) relative to low plasma osmolality 1, 3
- Typically >500 mOsm/kg in most SIADH cases 1, 2
- Urine osmolality exceeding plasma osmolality is a cardinal diagnostic feature 2, 6
Urine Sodium:
- Elevated urine sodium concentration (>20-40 mEq/L) 1, 6, 5
- Usually >40 mEq/L in most cases, reflecting continued natriuresis despite hyponatremia 6, 5
- Fractional excretion of sodium (FENa) >0.5% in approximately 70% of cases 5
- Important caveat: Urine sodium may be low (<30 mEq/L) in SIADH patients with poor oral intake or salt restriction 5
Clinical Assessment Requirements
Volume Status:
- Clinical euvolemia is essential for diagnosis - no edema, no orthostatic hypotension, normal skin turgor, moist mucous membranes 1, 2
- Absence of signs of volume depletion (distinguishes from hypovolemic hyponatremia) 2, 6
- Absence of edema or ascites (distinguishes from hypervolemic hyponatremia) 2, 6
Exclusion Criteria:
- Normal thyroid function (TSH) to exclude hypothyroidism 4, 2
- Normal adrenal function (cortisol) to exclude adrenal insufficiency 2, 6
- Normal renal function (creatinine) 2, 6
Diagnostic Algorithm
Step 1: Confirm hypotonic hyponatremia with serum sodium <135 mmol/L and plasma osmolality <275 mOsm/kg 3, 2
Step 2: Measure urine osmolality - if >100 mOsm/kg (typically >500 mOsm/kg), proceed with SIADH workup 1, 5
Step 3: Check urine sodium - if >20-40 mEq/L, this supports SIADH diagnosis 1, 6
Step 4: Assess clinical volume status - must demonstrate euvolemia without edema or volume depletion 1, 2
Step 5: Exclude other causes by checking TSH, cortisol, and confirming normal renal function 4, 2
Step 6: Supportive findings include low serum uric acid (<4 mg/dL), low BUN, and FENa >0.5% 4, 5
Common Diagnostic Pitfalls
Misinterpreting Volume Status:
- Failing to accurately assess euvolemia can lead to misdiagnosis, as cerebral salt wasting presents with hypovolemia while SIADH is euvolemic 4, 1
- In neurosurgical patients, distinguishing SIADH from cerebral salt wasting is critical since treatment approaches differ fundamentally 4, 1
Urine Sodium Interpretation:
- Low urine sodium (<30 mEq/L) does not exclude SIADH if the patient has poor oral intake 5
- A spot urine sodium/potassium ratio >1 correlates with 24-hour sodium excretion >78 mmol/day with approximately 90% accuracy 4
Overlooking Mild Hyponatremia: