Lab

Fractional Excretion of Bicarbonate

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Fractional Excretion of Bicarbonate, FE-HCO3

  • Indications
  1. Differentiate Proximal RTA from Distal RTA
  2. Requires that serum bicarbonate be normal
  • Calculation
  1. FE-HCO3 = (uHCO3 x sCr) / (sHCO3 x uCr)
  2. Annotation
    1. Where FE-HCO3 is Fractional Excretion of Bicarbonate
    2. Where uHCO3 is urine bicarbonate
    3. Where sHCO3 is serum bicarbonate
    4. Where uCr is Urine Creatinine
    5. Where sCr is Serum Creatinine
  • Interpretation
  1. FE-HCO3 <5%: Distal RTA
  2. FE-HCO3 >15%: Proximal RTA
    1. Assumes serum bicarbonate >20 meq/L