Calculators · Nephrology · Formula

Fractional excretion of sodium (FENa) and urea (FEUrea)

Also called FENa, FEUrea, FEUN

Separates prerenal azotaemia from acute tubular necrosis in oliguric AKI; FEUrea still works after diuretics.

mmol/L
mmol/L

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Formula and scoring

FENa = (U_Na × P_Cr) ÷ (P_Na × U_Cr) × 100. FEUrea = (U_urea × P_Cr) ÷ (P_urea × U_Cr) × 100. Units cancel if each pair uses the same unit.

Exam and ward pearls

  • Diuretics raise FENa: only 48% of prerenal patients on diuretics had FENa < 1%, while 89% had FEUrea ≤ 35% (Carvounis 2002).
  • Urea handling depends mainly on passive forces, so FEUrea is less affected by diuretics.

When not to rely on it

  • Validated in oliguric AKI; interpret cautiously in CKD and non-oliguric states.

Source

  1. Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-81. doi:10.1001/jama.236.6.579 · PMID 947239
  2. Carvounis CP, Nisar S, Guro-Razuman S. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int. 2002;62(6):2223-9. doi:10.1046/j.1523-1755.2002.00683.x · PMID 12427149

Formula and cut-offs checked against these sources on 8 Nov 2026. Educational tool for clinicians; it does not replace clinical judgement.

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