Fractional Excretion of Sodium (FENa) & Urea (FEUrea)

Distinguishes prerenal azotemia from intrinsic acute tubular necrosis (ATN) in acute kidney injury. FEUrea provides reliable interpretation when loop diuretics invalidate FENa.

FENa (%) = (U_Na × S_Cr) / (S_Na × U_Cr) × 100 FEUrea (%) = (U_Urea × S_Cr) / (S_Urea × U_Cr) × 100

Published Clinical Benchmark Cases

Clinical Parameters & Inputs

Urine Sodium (U_Na)Unit: mEq/L

mEq/L

Serum Sodium (S_Na)Unit: mEq/L

mEq/L

Urine Creatinine (U_Cr)Unit: mg/dL

mg/dL

Ensure matching units with serum creatinine (e.g. both in mg/dL or both in µmol/L).

Serum Creatinine (S_Cr)Unit: mg/dL

mg/dL

Patient Received Diuretics (Furosemide / Torsemide / Thiazide)?

Diuretics block tubular sodium reabsorption, causing falsely high FENa > 1% in prerenal states. Use FEUrea instead.

Urine Urea or Urea Nitrogen (U_Urea, optional)Unit: mg/dL

mg/dL

Serum Urea or BUN (S_Urea, optional)Unit: mg/dL

mg/dL

Computed OutputPrerenal Azotemia (FENa < 1%)

0.3% (FENa)

Diagnostic Interpretation

FENa is 0.3% (< 1.0%). Indicates intact, avid renal tubular sodium retention characteristic of Prerenal Azotemia (dehydration, hemorrhage, congestive heart failure, hepatorenal syndrome, or renal hypoperfusion). Kidneys are structurally sound.

Calculation Breakdown

Fractional Excretion of Sodium (FENa)

(15 × 2.5) / (140 × 90) × 100 =0.3%

Fractional Excretion of Urea (FEUrea)

(350 × 2.5) / (70 × 90) × 100 =13.9%

Clinical Decision Pearls

FENa < 1% is highly sensitive for prerenal azotemia in oliguric patients not receiving diuretics.
Exceptions where ATN can have FENa < 1%: Contrast-induced nephropathy, rhabdomyolysis / myoglobinuria, severe sepsis, acute glomerulonephritis.
When loop diuretics have been administered within 24-48 hours, FENa is falsely elevated; FEUrea < 35% is the validated gold-standard substitute.

Limitations & Confounders

Only validated in acute kidney injury with oliguria (urine output < 500 mL/day). Unreliable in non-oliguric or chronic stable renal disease.

Evidence & Citations

Miller TR, et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med. 1978;89(1):47-50.
Carvounis CP, et al. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int. 2002;62(6):2223-2229.