II. Indications
-
Renal Failure Assessment
- Prerenal Azotemia
- Acute Tubular Necrosis
III. Physiology
- Normal renal response to hypoperfusion (e.g. shock) is to retain Sodium
- Hence the Urine Sodium excretion (FENa) is reduced in Prerenal Failure (e.g. Dehydration)
- However, in intrinsic Renal Failure (e.g. ATN, AIN, GN), the Kidney loses its ability to retain Sodium
- Therefore, Sodium is wasted in the urine, cannot be reabsorbed, and results in a high FENa
IV. Calculation
- FENa = (Sodium Excretion x 100)/(total filtered load)
- Sodium Excretion = (Urine Sodium) / (Serum Sodium)
- Total filtered Load = (Urine Creatinine) / (Serum Creatinine)
- FENa = (uNa x sCr x 100) / (sNa x uCr)
- uNa is Urine Sodium
- sCr is Serum Creatinine
- sNa is Serum Sodium
- uCr is Urine Creatinine
V. Interpretation: Fractional Excretion of Sodium
- FENa <1%: Prerenal Azotemia
- Consistent with spot Urine Sodium <30 meq/L
- FENa >1-2%: Acute Intrinsic renal condition (e.g. Acute Tubular Necrosis)
- Consistent with spot Urine Sodium >30 meq/L
- FENa >4%: Post-Renal Azotemia
VI. Efficacy
- FENa can be high despite Prerenal Failure
- Diuretics increase FENa
- Delay FENa until 6-8 hours after last Diuretic dose
- Consider Fractional Excretion of Urea instead
- Diuretics increase FENa
- FENa may be low despite acute intrinsic renal disease
- Post-ischemic Acute Tubular Necrosis
- IV contrast or Hyperpigments
- Acute Glomerulonephritis
- Vasculitis
VII. Resources
- MDCalc Fractional Excretion of Sodium