Fractional Excretion Calculator
Calculate fractional excretion of sodium (FENa), urea (FEUrea), or uric acid to differentiate prerenal from intrinsic causes of acute kidney injury.
What is fractional excretion?
Fractional excretion (FE) measures the percentage of a filtered substance that is excreted in urine rather than reabsorbed by kidneys. Most commonly measured: FENa (sodium), FEUrea, FEUric Acid. Helps differentiate causes of acute kidney injury: prerenal (intact tubular function, low FE) vs intrinsic renal (damaged tubules, high FE). Requires simultaneous blood and urine samples. Essential tool for evaluating kidney function and AKI etiology.
How do I interpret FENa (Fractional Excretion of Sodium)?
FENa <1%: Suggests prerenal AKI (dehydration, heart failure, cirrhosis). Kidneys avidly retaining sodium, tubules functioning normally. FENa >2%: Suggests intrinsic renal disease (ATN, acute tubular necrosis). Damaged tubules cannot reabsorb sodium. FENa 1-2%: Indeterminate, consider clinical context. Limitations: Diuretics invalidate FENa (use FEUrea instead), chronic kidney disease may have higher baseline. Most useful in oliguric AKI without diuretic use.
When should I use FEUrea instead of FENa?
Use FEUrea when: Patient recently received diuretics (diuretics artificially elevate FENa), chronic kidney disease present, need more reliable prerenal assessment. FEUrea <35%: Prerenal AKI. FEUrea >50%: Intrinsic renal disease (ATN). FEUrea less affected by diuretics because urea reabsorption is passive. More reliable than FENa in diuretic-treated patients. Both tests complementary; use FEUrea if FENa unreliable.
What samples do I need for fractional excretion calculation?
Requires 4 values: Urine substance concentration, plasma/serum substance concentration, urine creatinine, plasma/serum creatinine. Best practice: Collect spot urine sample and draw blood simultaneously for accurate comparison. Units must match (mg/dL or mmol/L). Random urine sample acceptable (no 24-hour collection needed). Ensure patient not on diuretics for FENa accuracy. Calculate creatinine clearance simultaneously to assess GFR.
What causes high or low fractional excretion?
Low FE (<1% sodium, <35% urea): Prerenal causes - dehydration, CHF, cirrhosis, sepsis with preserved kidney function, early obstruction. Kidneys trying to conserve substance. High FE (>2% sodium, >50% urea): Intrinsic renal - ATN (acute tubular necrosis), acute interstitial nephritis, glomerulonephritis, established obstruction. Tubules damaged, cannot reabsorb. Also high in diuretic use, salt-wasting nephropathy, adrenal insufficiency. Clinical context essential for interpretation.