Fractional Excretion of Bicarbonate Calculator
Fractional Excretion of Bicarbonate Calculator
Separate proximal from distal renal tubular acidosis by measuring how much filtered bicarbonate is lost — valid only during bicarbonate loading, once plasma bicarbonate has been normalised.
Fractional Excretion of Bicarbonate
HCO3 clearance ratioUrine bicarbonate 60 mmol/L, plasma bicarbonate 22 mmol/L, urine creatinine 15 mg/dL, plasma creatinine 1.0 mg/dL
Formula
- urine HCO3, plasma HCO3
- both in mmol/L — the two bicarbonate values must be in the same units as each other
- urine creatinine, plasma creatinine
- both in mg/dL, or both in µmol/L; the units cancel, so they must match
- bicarbonate loading
- the test is only interpretable with the plasma bicarbonate raised into the normal range, typically by oral or intravenous sodium bicarbonate
- sample handling
- urine must be fresh and analysed promptly, as carbon dioxide is lost on standing and the measured bicarbonate falls
Worked example
Urine bicarbonate 60 mmol/L, plasma bicarbonate 22 mmol/L, urine creatinine 15 mg/dL, plasma creatinine 1.0 mg/dL
60 × 1.0 = 60
22 × 15 = 330
60 ÷ 330 × 100 = 18.2%
Above 15% → consistent with proximal (type 2) RTA
Separating the renal tubular acidoses
| Type 1 (distal) | Type 2 (proximal) | Type 4 | |
|---|---|---|---|
| Urine pH during acidosis | > 5.5, inappropriately high | < 5.5 once plasma HCO3 is below threshold | Usually < 5.5 |
| Serum potassium | Low | Low | High |
| Urine anion gap | Positive | Positive | Positive |
| FEHCO3 during bicarbonate loading | < 5% | > 15% | < 5% |
Why the preparation matters
| Condition of the sample | What FEHCO3 shows |
|---|---|
| Untreated, plasma HCO3 low | Near zero in every type of RTA — uninterpretable |
| Plasma HCO3 raised into the normal range | Distal RTA < 5%, proximal RTA > 15% |
| Urine left standing before analysis | Falsely low, as carbon dioxide is lost from the specimen |
Why this test only works during bicarbonate loading
The fractional excretion of bicarbonate compares how much of the filtered bicarbonate load appears in the urine with how much of the filtered creatinine does, expressed as a percentage. Its purpose is narrow but genuinely useful: it separates proximal, type 2 renal tubular acidosis, in which the proximal tubule cannot reabsorb bicarbonate, from distal, type 1 disease, in which bicarbonate handling is intact and the defect lies in the secretion of acid by the collecting duct.
The essential point is that the test is only interpretable during bicarbonate loading, with the plasma bicarbonate brought up into the normal range. Measured on an untreated acidotic patient the fractional excretion is near zero whatever the diagnosis, because the plasma bicarbonate sits below even a reduced reabsorptive threshold and there is very little filtered bicarbonate available to lose. A low value on an unprepared sample therefore means nothing at all, and reading it as evidence of distal disease is the commonest way the test is misused.
Once the plasma bicarbonate has been raised, the two patterns separate cleanly. The proximal tubule in type 2 disease has a lowered threshold, so as soon as the plasma level rises above it bicarbonate spills into the urine and the fractional excretion exceeds 15%, often reaching 15 to 20% or more. In type 1 disease the proximal tubule reabsorbs normally and the fractional excretion stays below 5%. Neither end of that range is the normal one: both are diagnoses, and it is the middle ground that supports neither and calls for the clinical context and a check that the loading was adequate.
Two technical requirements are easy to overlook. The urine and plasma creatinine must be in the same units, and the two bicarbonate values likewise, since the units cancel within each pair; mixing mg/dL with µmol/L produces a result wrong by a factor of nearly ninety. And the urine must be collected fresh and analysed promptly, because carbon dioxide escapes from a specimen left standing and the measured bicarbonate falls, biasing the result towards a falsely distal pattern.
Frequently asked questions
How is the fractional excretion of bicarbonate calculated?
Multiply urine bicarbonate by plasma creatinine, divide by plasma bicarbonate multiplied by urine creatinine, and multiply by 100. The creatinines must be in the same units as each other, as must the two bicarbonates.
Why must the patient be bicarbonate-loaded first?
Because an untreated acidotic patient has little filtered bicarbonate to lose, so the fractional excretion is near zero in every type of RTA. The plasma bicarbonate must be brought into the normal range before the result means anything.
What FEHCO3 indicates proximal RTA?
Above 15% during bicarbonate loading. The proximal tubule has a reduced reabsorptive threshold, so bicarbonate spills into the urine as soon as the plasma level rises above it.
What FEHCO3 indicates distal RTA?
Below 5% during bicarbonate loading. In type 1 disease bicarbonate reabsorption is normal and the defect is in distal hydrogen ion secretion instead.
Why does the urine sample need to be analysed quickly?
Carbon dioxide is lost from urine left standing, so the measured bicarbonate falls. A delayed sample gives a falsely low fractional excretion, biasing the result towards a distal pattern.
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References
- Rodríguez Soriano J. Renal tubular acidosis: the clinical entity. J Am Soc Nephrol. 2002;13(8):2160–2170.
- Rose BD, Post TW. Clinical Physiology of Acid-Base and Electrolyte Disorders. 5th ed. McGraw-Hill; 2001.