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Fractional Excretion of SodiumAka: FENa

Indications
1. Renal Failure Assessment
  1. Prerenal Azotemia
  2. Acute Tubular Necrosis
Formula
1. FENa = (Sodium Excretion x 100)/(total filtered load)
  1. Sodium Excretion = (Urine Sodium) / (Serum Sodium)
  2. Total Load = (Urine Creatinine) / (Serum Creatinine)
2. FENa = (uNa x sCr x 100) / (sNa x uCr)
Interpretation of Fractional Excretion of Sodium
1. FENa <1%: Prerenal Azotemia
  1. Consistent with spot Urine Sodium <30 meq/L
2. FENa >2%: Acute Tubular Necrosis (ATN)
  1. Consistent with spot Urine Sodium >30 meq/L
Efficacy
1. FENa invalidated by Diuretics
  1. Delay FENa until 6-8 hours after last Diuretic dose
2. FENa can be low despite Acute Tubular Necrosis
  1. Post-ischemic Acute Tubular Necrosis
  2. IV contrast or Hyperpigments
  3. Acute Glomerulonephritis
  4. Vasculitis

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This page was written by Scott Moses, MD. This page was last revised before 5/10/08, prior to consistent tracking of revision date. This page was last published on 11/3/2008. Please send comments and questions to the editor. Contact information may be found on the page About the Family Practice Notebook. These pages are best viewed with Microsoft Internet Explorer 6.0.