nephrology & metabolismformula

Fractional Excretion of Sodium (FENa)

FENa calculates the percentage of filtered sodium excreted in urine, helping differentiate prerenal AKI (sodium avidly retained, FENa <1%) from intrinsic renal injury (FENa >3%). Formula: (Urine Na × Serum Cr) / (Serum Na × Urine Cr) × 100.

inputs

mmol/L
mmol/L

✓ when to use

Use in the evaluation of acute kidney injury (AKI) to help distinguish prerenal azotaemia from acute tubular necrosis (ATN) or other intrinsic renal causes. Most useful in oliguric AKI where the distinction affects immediate management (volume resuscitation vs avoidance of fluid overload).

✗ when not to use

FENa is UNRELIABLE in patients receiving diuretics (increases urinary sodium regardless of volume status) — use fractional excretion of urea (FEUrea) instead. Also unreliable in CKD (chronically impaired sodium reabsorption), contrast nephropathy (FENa can be low), rhabdomyolysis, and early obstruction. FENa is a snapshot — a single value in a dynamic clinical picture.

clinical pearls

  • If the patient is on diuretics, FENa is unreliable — use FEUrea instead. FEUrea <35% suggests prerenal, >50% suggests intrinsic renal. Urea reabsorption is less affected by diuretics than sodium reabsorption.
  • FENa must use simultaneous serum and urine samples. A delayed urine sample after fluid resuscitation may give misleadingly high values.
  • A low FENa (<1%) does not always mean prerenal. Some intrinsic renal conditions also produce low FENa: contrast nephropathy, early obstruction, rhabdomyolysis, acute glomerulonephritis, and some cases of AIN.
  • In the context of AKI, FENa is one piece of the puzzle. Combine with clinical history (volume status, nephrotoxin exposure), urinalysis (casts, proteinuria), and renal ultrasound for a complete assessment.
  • Ensure units are consistent — serum and urine creatinine must be in the same units for the formula to work correctly. Unit mismatch is a common calculation error.

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ckd-epi-2021eGFR assessmentkdigo-akiAKI staginganion-gapMetabolic assessment in AKI