Lab

Fractional Excretion of Sodium

search

Fractional Excretion of Sodium, FENa

  • See Also
  1. Sodium and Water Homeostasis
  2. Hyponatremia
  3. Hypernatremia
  4. Serum Sodium
  5. Total Body Sodium Deficit
    1. Includes Total Body Water and Free Water Deficit Calculations
  6. Serum Osmolality
  7. Osmolal Gap
  8. Urine Sodium
  • Indications
  1. Renal Failure Assessment
    1. Prerenal Azotemia
    2. Acute Tubular Necrosis
  • 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
  • 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
  • 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
  • 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
  • Resources
  1. MDCalc Fractional Excretion of Sodium
    1. http://www.mdcalc.com/fractional-excretion-of-sodium-fena/
  • References
  1. Braun (2015) Am Fam Physician 91(5): 299-307 [PubMed]