The Cardiovascular
ToolsCalculators

Fractional excretion of sodium (FENa)

Distinguishes prerenal from intrinsic acute kidney injury.

Fractional excretion of sodium (FENa)
Urine sodium
mEq/L
Serum sodium
mEq/L
Urine creatinine
mg/dL
Serum creatinine
mg/dL
Complete the fields above to see the result.

When to use

  • Differentiating prerenal azotaemia from acute tubular necrosis in oliguric acute kidney injury.

Formula

FENa = (urine Na × serum Cr) / (serum Na × urine Cr) × 100.

Pearls and pitfalls

  • Diuretics invalidate FENa — use the fractional excretion of urea (FEurea, <35 % suggests prerenal) instead.
  • A low FENa also occurs in contrast nephropathy, early obstruction, glomerulonephritis and hepatorenal syndrome.

References

  1. Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579–81.