What is a normal FENa?

FENa 2% suggests intrinsic renal disease (acute tubular necrosis). FENa 1–2% is indeterminate and requires clinical correlation.

When is FENa unreliable?

FENa is unreliable in patients receiving diuretics, as diuretics increase sodium excretion regardless of volume status. In these cases, fractional excretion of urea (FEUrea) is preferred. FENa is also less reliable in chronic kidney disease, contrast nephropathy, rhabdomyolysis, and early obstruction.

What is the difference between FENa and FEUrea?

FEUrea (fractional excretion of urea) is not significantly affected by diuretics, making it useful when FENa is unreliable. FEUrea 50% suggests intrinsic renal disease. FEUrea should be used in diuretic-treated patients and as a complementary test to FENa.

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FENa Calculator

Calculate FENa (fractional excretion of sodium) to help differentiate prerenal azotemia from intrinsic renal acute kidney injury. Enter urine and serum sodium and creatinine values.

Inputs

Urine Sodium (mEq/L)

Plasma Sodium (mEq/L)

Urine Creatinine (mg/dL)

Plasma Creatinine (mg/dL)

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Enter all four values to calculate FENa.

Prerenal< 1%

Intrinsic Renal> 2%

Indeterminate1–2%

Formula: FENa (%) = (U_Na × P_Cr) / (P_Na × U_Cr) × 100

Disclaimer: For educational purposes only. Not a substitute for clinical judgment.

When to use

Use FENa to differentiate prerenal azotemia (volume depletion, heart failure, cirrhosis) from intrinsic renal injury (ATN) in patients with acute kidney injury (AKI). Collect urine and blood specimens simultaneously before giving diuretics.

Clinical pearls

FENa is unreliable in patients receiving diuretics — use FEUrea (< 35% = prerenal) instead.
FENa < 1% can be seen in prerenal AKI AND in some intrinsic causes: contrast nephropathy, myoglobinuria, early obstruction, and acute glomerulonephritis.
FENa > 2% on CKD baseline does not necessarily indicate ATN — chronic kidney disease alters sodium handling.
Specimens should be collected before IV fluids or diuretics are administered for best accuracy.

References

Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-581.
Steiner RW. Interpreting the fractional excretion of sodium. Am J Med. 1984;77(4):699-702.

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About This Tool

What Is FENa and How Is It Calculated?

The Fractional Excretion of Sodium (FENa) is a calculated value that compares the amount of sodium filtered by the kidneys with the amount actually excreted in the urine. It is expressed as a percentage and is calculated as: FENa = (Urine Na × Plasma Cr) / (Plasma Na × Urine Cr) × 100. FENa is one of the most widely used diagnostic tools for differentiating prerenal acute kidney injury (where the kidney is functioning normally but underperfused) from intrinsic renal injury (where tubular damage impairs sodium reabsorption).

FENa Interpretation: What Do the Results Mean?

In prerenal AKI, the kidneys are underperfused but tubular function is intact — the tubules avidly reabsorb sodium, resulting in FENa < 1%. In intrinsic renal failure (particularly acute tubular necrosis, ATN), damaged tubules cannot reabsorb sodium efficiently, leading to FENa > 2%. Values between 1–2% are indeterminate and require clinical correlation. It's important to note that FENa has significant limitations: it is unreliable in patients who have received diuretics (which increase sodium excretion regardless of volume status), and several intrinsic renal diseases can present with low FENa, including contrast-induced nephropathy, rhabdomyolysis, acute glomerulonephritis, and early urinary tract obstruction.

FENa vs. FEUrea

When patients have received diuretics, the Fractional Excretion of Urea (FEUrea) is a more reliable alternative because urea reabsorption is not significantly affected by most diuretics. A FEUrea < 35% suggests prerenal etiology, while > 50% suggests intrinsic renal disease. In clinical practice, both FENa and FEUrea should be interpreted in the context of the overall clinical picture, including volume status, medication history, and urine sediment analysis.

🔑 Clinical Pearls

Always collect urine and blood samples simultaneously BEFORE administering diuretics or IV fluids.
FENa < 1% with diuretic use is meaningless — use FEUrea instead.
Low FENa (< 1%) does not always mean prerenal — contrast nephropathy, rhabdomyolysis, and acute GN can present similarly.
In chronic kidney disease, baseline FENa may be > 1% due to obligate sodium loss — interpret in context.

Key References

Espinel CH. The FENa test. JAMA. 1976;236(6):579-581.
Steiner RW. Interpreting the fractional excretion of sodium. Am J Med. 1984;77(4):699-702.
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.

Formula last verified: February 2026