Urine Studies Consistent with SIADH
The diagnostic urine studies consistent with SIADH include urine osmolality >500 mOsm/kg and urine sodium concentration >20-40 mEq/L in the setting of euvolemic hyponatremia. 1
Key Diagnostic Criteria for SIADH
SIADH diagnosis requires the following laboratory findings:
Serum findings:
- Serum sodium <134 mEq/L
- Plasma osmolality <275 mOsm/kg
- Clinical euvolemia (no edema, normal vital signs)
Urine findings:
Distinguishing SIADH from Other Causes of Hyponatremia
The differentiation between SIADH and other causes of hyponatremia, particularly Cerebral Salt Wasting (CSW), is critical as treatment approaches differ significantly:
Volume status assessment:
- SIADH: Euvolemic with normal vital signs, no edema
- CSW: Hypovolemic with orthostatic hypotension, tachycardia 1
Central Venous Pressure (CVP):
- SIADH: CVP 6-10 cm H₂O
- CSW: CVP <6 cm H₂O 3
Additional Laboratory Parameters
Several other laboratory parameters can help confirm SIADH diagnosis:
- Serum uric acid: Typically low (<4 mg/dL) in SIADH with a positive predictive value of 73-100% 3, 4
- Blood urea nitrogen (BUN): Usually low in SIADH 4
- Anion gap: Lower anion gap with nearly normal total CO₂ and serum potassium despite dilution 4
- Fractional excretion of sodium: High (>0.5%) in approximately 70% of SIADH cases 4
Clinical Pitfalls and Caveats
- Physical examination alone is insufficient for determining extracellular fluid status, with sensitivity of only 41.1% and specificity of 80% 3
- Urine sodium may be low (<30 mmol/L) in SIADH patients with poor nutritional intake, which can lead to misdiagnosis 4
- ADH levels have limited diagnostic value as SIADH has been documented in patients with undetectable ADH 3
- Always exclude other causes of euvolemic hyponatremia (hypothyroidism, adrenal insufficiency) before confirming SIADH diagnosis 2, 5
- Consider "reset osmostat" as a variant of SIADH when urine osmolality is lower than typically seen in classic SIADH 4, 6
Patterns of Urine Osmolality in SIADH
Urine osmolality patterns can help guide treatment decisions:
- Patients with initial urine osmolality <400 mOsm/kgH₂O may respond to water restriction alone 7
- Patients with high urine osmolality (>600 mOsm/kg) often require more aggressive treatment and may be good candidates for V2 receptor antagonists 4
By systematically evaluating these urine studies alongside clinical assessment and serum parameters, clinicians can accurately diagnose SIADH and distinguish it from other causes of hyponatremia.