Fractional Excretion of Sodium

Urine Na
Serum Na
Urine Cr
Serum Cr
Decimal Precision:
FENa = (UrineNa / SerumNa) / (UrineCr / SerumCr) * 100
  • The FENa helps differentiate acute tubular necrosis from a prerenal state.
  • FENa > 2 % usually indicates ATN.
  • FENa < 1% usually indicates a prerenal state such as dehydration or other forms of prerenal azotemia.
  • FENa's between 1% and 2% may be seen with either ATN or prerenal state.
  1. Stein JH, ed. Internal Medicine. 4th edition. Mosby-Year Book. 1994.
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