The bicarbonate deficit calculator estimates how much bicarbonate must be replaced to raise a patient’s serum HCO₃ toward a target value in severe metabolic acidosis. It is a planning tool for IV sodium bicarbonate dosing in nephrology and emergency medicine.
How it works
The deficit in millimoles is the apparent distribution volume of bicarbonate multiplied by the gap between the target and measured concentration:
Deficit (mmol) = factor × weight(kg) × (target HCO3 − measured HCO3)The factor is the fraction of body weight that approximates the bicarbonate
space — conventionally 0.4 to 0.5. Because bicarbonate is buffered across
cells and bone, the apparent space expands as acidosis worsens, so a higher
factor (up to about 0.8) is sometimes used when the measured HCO₃ is very low.
One ampoule of 8.4% sodium bicarbonate contains about 50 mmol in 50 mL, which
helps translate the deficit into a number of ampoules.
Example and notes
For a 70 kg patient with a measured HCO₃ of 10 mmol/L and a target of 18 mmol/L,
using a factor of 0.5: 0.5 × 70 × (18 − 10) = 280 mmol, roughly 5.6 ampoules of
8.4% solution.
Replace only part of the deficit (often half), then recheck the blood gas and recalculate. Rapid full correction risks overshoot, hypokalaemia, hypocalcaemia, sodium and volume overload, and paradoxical CSF acidosis. Treating the underlying cause is usually preferred in lactic acidosis and ketoacidosis.
Choosing the right distribution factor
The factor that best reflects the bicarbonate space depends on the clinical situation:
| Setting | Recommended factor |
|---|---|
| Mild to moderate acidosis (HCO₃ 12–18 mmol/L) | 0.4 to 0.5 |
| Severe acidosis (HCO₃ 8–12 mmol/L) | 0.5 to 0.6 |
| Very severe / near-zero HCO₃ | Up to 0.8 |
Higher factors are used when acidosis is severe because intracellular buffers and bone release bicarbonate, expanding the apparent distribution space. If the initial correction undershoots, a repeat calculation with the new measured HCO₃ will guide the next increment.
Practical workflow
- Confirm the acidosis type — normal-anion-gap (hyperchloraemic, diarrhoea, renal tubular acidosis) responds well to bicarbonate; high-anion-gap (lactic, keto) is better treated by addressing the cause.
- Calculate the full deficit using this tool.
- Administer only half the calculated dose as a slow infusion over one to two hours.
- Recheck arterial blood gas and electrolytes, paying attention to potassium (which shifts intracellularly as pH rises) and ionised calcium (which falls as pH rises).
- Recalculate and give a further increment if needed.
One ampoule of 8.4% sodium bicarbonate (50 mL) contains approximately 50 mmol. For an 8.4% solution, the sodium load is also approximately 50 mmol of sodium per ampoule — relevant in patients with heart failure or where sodium overload is a concern. A less concentrated 1.26% or 1.4% solution delivers the same bicarbonate more slowly with greater volume and less sodium per dose.