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Fractional Excretion of Sodium (FENa)

FENa = (urine Na × plasma creatinine) ÷ (plasma Na × urine creatinine) × 100. In oliguric AKI, FENa <1% suggests a prerenal cause and >2% suggests acute tubular necrosis; diuretics invalidate it (use FEUrea).

Serum sodium (mmol/L or mEq/L)
Urine sodium (mmol/L or mEq/L)
Serum creatinine (mg/dL or µmol/L)
Urine creatinine (mg/dL or µmol/L or mmol/L)

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Interpretation

Result	Meaning
< 1%	Prerenal
1–2%	Indeterminate
> 2%	Intrinsic (ATN)

Formula

FENa (%) = (U_Na × P_Cr) ÷ (P_Na × U_Cr) × 100 (creatinine in the same units for plasma and urine).

Clinical pearls

Diuretics raise FENa even in prerenal states; use fractional excretion of urea instead.
Low FENa (<1%) also occurs in contrast nephropathy, rhabdomyolysis, early obstruction, glomerulonephritis and hepatorenal syndrome.
Validated in oliguric AKI; less discriminating in non-oliguric AKI and CKD.

Examples

fena serum na 140 urine na 20 serum cr 2 urine cr 100 fractional excretion of sodium: Na 135, UNa 40, creatinine 250 umol/L, urine creatinine 5 mmol/L fena una 15 pna 138 cr 180 umol ucr 9 mmol/L

FAQ

What FENa indicates prerenal AKI?

Below 1% in an oliguric patient not on diuretics suggests a prerenal cause; above 2% suggests ATN.

Can FENa be used if the patient got furosemide?

No, diuretics increase sodium excretion; fractional excretion of urea (<35% prerenal) is preferred.

Evidence

Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA 1976;236:579–81.

Nephrology & Electrolytes · Reviewed 2026-10-14 · Methodology