The Fractional Excretion of Urea (FEUrea) serves the exact same diagnostic purpose as the FENa, but it acts as the vital fallback tool when a patient has been administered diuretics.
The Diuretic Conundrum
Imagine a patient arrives at the hospital with heart failure. They are given a massive dose of intravenous furosemide (a loop diuretic) to clear fluid from their lungs. The next day, their creatinine spikes. You want to know if they have pre-renal AKI (perhaps you diuresed them too much, causing hypovolemia) or intrinsic AKI (Acute Tubular Necrosis).
If you check a FENa, it will be >2%. The diuretic is literally forcing the kidney to waste sodium. You might misdiagnose them with ATN.
The Utility of Urea
Unlike sodium, urea reabsorption in the proximal tubule is largely unaffected by loop diuretics. Therefore, if the patient is hypovolemic (pre-renal), the kidney will still aggressively reabsorb urea to help reabsorb water, pulling the FEUrea down.
FEUrea (%) = [(Urine BUN × Serum Creatinine) / (Serum BUN × Urine Creatinine)] × 100
Interpretation
- < 35%: Suggests pre-renal AKI (hypovolemia, low effective circulating volume).
- > 50%: Suggests intrinsic AKI (ATN).