Calculates fractional bicarbonate excretion from paired urine and serum bicarbonate and creatinine measurements. This premium guide explains physiology, calculation, interpretation, limitations and evidence.
Overview
Calculates fractional bicarbonate excretion from paired urine and serum bicarbonate and creatinine measurements.
Clinical Significance
Used mainly during supervised bicarbonate loading to assess proximal bicarbonate wasting.
When to Use
- Support specialist evaluation of proximal renal tubular acidosis.
- Use paired samples under protocol conditions.
- Interpret with urine pH and serum bicarbonate.
How It Is Calculated
- FEHCO₃ = (urine HCO₃ × plasma creatinine) ÷ (plasma HCO₃ × urine creatinine) × 100.
- MedicalC validates required values and rounds only the displayed result.
Interpretation
- Used mainly during supervised bicarbonate loading to assess proximal bicarbonate wasting.
- Review source measurements, collection quality and clinical trajectory as well as the final value.
Worked Example
Enter clinically plausible values in the displayed units. MedicalC applies: FEHCO₃ = (urine HCO₃ × plasma creatinine) ÷ (plasma HCO₃ × urine creatinine) × 100. Verify the result against the source data before use.
Patient Considerations
- Consider kidney function, diuretics, intravenous fluids, nutrition, body composition and rapidly changing physiology.
- For timed urine calculations, confirm collection duration, volume and completeness.
- For electrolyte prescriptions, include all oral, enteral, intravenous and medication sources.
Limitations
- Routine spot testing is often unhelpful.
- Urine bicarbonate handling is technically difficult.
- Reduced GFR complicates interpretation.
Clinical Pearls
- Use paired serum and urine values obtained as close together as possible when the formula requires them.
- A derived renal index is only as reliable as the collection and assays used.
- Check whether treatment occurred between sampling and interpretation.
Common Mistakes
- Mixing µmol/L and mmol/L creatinine values.
- Using spot concentrations as daily excretion rates.
- Ignoring incomplete timed urine collection.
- Treating a maintenance estimate as a replacement or emergency-correction regimen.
- Using a derived acid–base value without checking albumin, lactate and the blood gas.
Evidence Base
Principal source: Batlle D, et al. Distal renal tubular acidosis and the potassium enigma. Semin Nephrol. 2019.
Frequently Asked Questions
What does the Fractional Excretion of Bicarbonate calculator do?
Calculates fractional bicarbonate excretion from paired urine and serum bicarbonate and creatinine measurements.
Can this result establish a diagnosis?
No. It is an adjunct that must be combined with clinical assessment and current guidance.
Why are timing and units important?
Derived results can be misleading when values are non-contemporaneous or use incompatible units.
When should the calculation be repeated?
Repeat when treatment, kidney function or the relevant measured values change and a new result could influence care.
Does this replace a specialist protocol?
No. Direct measurement, laboratory interpretation and specialist protocols take priority.
References
- Batlle D, et al. Distal renal tubular acidosis and the potassium enigma. Semin Nephrol. 2019. — Primary publication, guideline or authoritative specialty source.
Reviewed by: MedicalC Clinical Editorial Team
Last reviewed: July 2026