II. Indications

III. Physiology

  1. Normal renal response to hypoperfusion (e.g. shock) is to retain Sodium
    1. Hence the Urine Sodium excretion (FENa) is reduced in Prerenal Failure (e.g. Dehydration)
  2. However, in intrinsic Renal Failure (e.g. ATN, AIN, GN), the Kidney loses its ability to retain Sodium
    1. Therefore, Sodium is wasted in the urine, cannot be reabsorbed, and results in a high FENa

IV. Calculation

  1. FENa = (Sodium Excretion x 100)/(total filtered load)
    1. Sodium Excretion = (Urine Sodium) / (Serum Sodium)
    2. Total filtered Load = (Urine Creatinine) / (Serum Creatinine)
  2. FENa = (uNa x sCr x 100) / (sNa x uCr)
    1. uNa is Urine Sodium
    2. sCr is Serum Creatinine
    3. sNa is Serum Sodium
    4. uCr is Urine Creatinine

V. Interpretation: Fractional Excretion of Sodium

  1. FENa <1%: Prerenal Azotemia
    1. Consistent with spot Urine Sodium <30 meq/L
  2. FENa >1-2%: Acute Intrinsic renal condition (e.g. Acute Tubular Necrosis)
    1. Consistent with spot Urine Sodium >30 meq/L
  3. FENa >4%: Post-Renal Azotemia

VI. Efficacy

  1. FENa can be high despite Prerenal Failure
    1. Diuretics increase FENa
      1. Delay FENa until 6-8 hours after last Diuretic dose
      2. Consider Fractional Excretion of Urea instead
  2. FENa may be low despite acute intrinsic renal disease
    1. Post-ischemic Acute Tubular Necrosis
    2. IV contrast or Hyperpigments
    3. Acute Glomerulonephritis
    4. Vasculitis

VII. Resources

  1. MDCalc Fractional Excretion of Sodium
    1. http://www.mdcalc.com/fractional-excretion-of-sodium-fena/

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