Metabolic acidosis is characterised by a primary fall in serum bicarbonate with low blood pH, commonly due to lactic acidosis, ketoacidosis, renal failure, or gastrointestinal or renal bicarbonate loss. In patients where intravenous sodium bicarbonate has been judged appropriate, an estimate of the total body bicarbonate deficit can help plan the initial replacement strategy.

Bedside deficit equations approximate the amount of base required to raise serum bicarbonate from the measured value to a chosen target, using an assumed bicarbonate distribution volume. This calculator uses 0.4 L/kg and a fixed target serum bicarbonate of 24 mmol/L, which aligns with commonly described adult formulas, although other protocols use distribution factors between about 0.3 and 0.5 L/kg or aim only for partial correction. The estimate is therefore approximate and may be less reliable at extremes of body size or in marked fluid overload or depletion.

Physiological and clinical reviews highlight that evidence for intravenous sodium bicarbonate in acute metabolic acidosis is limited and mixed. Alkali therapy is generally reserved for more severe acidaemia or situations where acidosis contributes to haemodynamic instability, arrhythmia risk, life-threatening hyperkalaemia or certain toxic ingestions, rather than for modest, isolated reductions in bicarbonate. Trials in lactic acidosis and diabetic ketoacidosis have not shown consistent haemodynamic or mortality benefit and raise concerns about sodium and volume load, hypernatraemia, carbon dioxide generation and potential paradoxical intracellular or cerebral acidosis. In the BICAR-ICU trial of critically ill adults with severe metabolic acidaemia, an isotonic bicarbonate infusion protocol did not improve the primary composite outcome overall, but reduced the need for renal replacement therapy and suggested a survival benefit in the predefined subgroup with acute kidney injury.

Accordingly, this bicarbonate deficit estimate should be used as a rough dosing guide only, interpreted alongside contemporaneous blood gases (pH, PaCO₂, anion gap, lactate/ketones), renal function and local protocols, and not as the sole basis for deciding whether to administer bicarbonate.

Bicarbonate Deficit Estimation in Metabolic Acidosis