FENa (Fractional Excretion of Sodium)

Prerenal vs intrinsic acute kidney injury — not valid after loop diuretics

Urine sodium (mEq/L)

Serum sodium (mEq/L)

Urine creatinine

Serum creatinine

Enter all four values to calculate

Evidence

Original publication

Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. In oliguric acute renal failure a fractional sodium excretion below 1% pointed to a prerenal cause and above 1% (usually well above) to acute tubular necrosis.

JAMA, 1976

Open source

Validation

Steiner RW. Interpreting the fractional excretion of sodium. Reviews the test’s diagnostic limits — it is unreliable after loop diuretics, in chronic kidney disease, contrast nephropathy, rhabdomyolysis, and acute glomerulonephritis, where FEUrea may be preferred.

American Journal of Medicine, 1984

Open source

Last reviewed: 2026-09

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