Interpreting Fractional Excretion of Urea (FEUrea) in Suspected SIADH
A low FEUrea (<35-40%) suggests hypovolemia and argues against SIADH, while a high FEUrea (>55%) supports SIADH but must be interpreted in the context of age, salt intake, and volume status to distinguish it from cerebral salt wasting (CSW).
Key Diagnostic Framework
The interpretation of FEUrea in suspected SIADH requires understanding that extracellular fluid (ECF) status is the key to distinguishing between SIADH and CSW 1. However, physical examination alone has poor sensitivity (41.1%) for determining volume status 1.
FEUrea Values and Their Meaning
FEUrea >55%: Associated with SIADH, but this finding is inversely correlated with salt intake 2
- Patients with SIADH on low sodium intake may have lower FEUrea despite true SIADH
- When salt intake increases, FEUrea decreases in SIADH patients 2
FEUrea <35-40%: Suggests saline responsiveness and indicates true hypovolemia (CSW or other volume depletion) rather than SIADH 1
Critical Age-Related Considerations
Age significantly affects FEUrea interpretation 3:
- Young patients with SIADH: FEUrea typically 58 ± 14% with plasma urea 18 ± 8 mg/dL
- Elderly patients with SIADH: FEUrea only 44 ± 15% with plasma urea 29 ± 8 mg/dL
- Elderly control subjects without SIADH have FEUrea of 36 ± 9%
This means frankly elevated FEUrea values should only be expected in young patients with SIADH. In elderly patients, FEUrea may be in the "normal" range despite true SIADH 3.
Algorithmic Approach to Interpretation
Step 1: Assess Volume Status Beyond Physical Exam
- Physical exam is insufficient (sensitivity 41.1%) 1
- Consider CVP if available:
- CVP 6-10 cm H₂O = normovolemic (SIADH)
- CVP <6 cm H₂O = hypovolemic (CSW)
- CVP <5 cm H₂O = definite hypovolemia 1
Step 2: Interpret FEUrea in Context
If FEUrea is LOW (<35-40%):
- This indicates saline responsiveness 1
- Suggests true hypovolemia (CSW, not SIADH)
- Consider saline challenge: 71-100% positive predictive value for sodium correction with 0.9% saline 1
If FEUrea is HIGH (>55%):
- Consider patient age:
- Assess salt intake:
Step 3: Use Complementary Laboratory Values
- SIADH typically shows LOW plasma urea
- Young patients with SIADH: ~18 mg/dL
- Elderly patients with SIADH: ~29 mg/dL (vs. 39 mg/dL in elderly controls)
- Serum uric acid <4 mg/dL has 73-100% positive predictive value for SIADH
- More reliable than urea, especially in elderly patients
- Present in 70% of SIADH cases vs. 40% of salt-depleted patients
- Urine Na >30 mEq/L supports SIADH
- Fractional excretion of sodium >0.5% in 70% of SIADH cases
- Caveat: Low urine sodium can occur in SIADH with poor oral intake
Common Pitfalls to Avoid
Don't rely on FEUrea alone in elderly patients - values overlap significantly with normal aging 3
Don't ignore dietary sodium intake - low salt intake can mask high FEUrea in SIADH 2
Don't use physical exam as sole volume assessment - it has only 41% sensitivity 1
Don't assume high FEUrea always means SIADH - must exclude CSW using volume status assessment 1
Don't forget the dissociation between urea and uric acid - uric acid clearance is NOT dependent on sodium excretion, making it more reliable than FEUrea 2
Practical Clinical Application
When FEUrea is elevated in suspected SIADH:
- Measure serum uric acid (most specific single test)
- Assess actual volume status (CVP if available, or saline challenge)
- Consider patient age and adjust expectations accordingly
- Evaluate dietary sodium intake history
- Measure urine sodium and fractional excretion of sodium
The combination of high FEUrea + low serum uric acid + normovolemia provides the strongest evidence for SIADH over CSW 1, 4, 5.