The base excess is a calculated figure which provides an estimate of the metabolic component of the acid-base balance.
The base excess is defined as the amount of H+ ions that would be required to return the pH of the blood to 7.35 if the pCO2 were adjusted to normal.
Because the base excess is a calculated (not a measured) value, it may be inaccurate and misleading. Despite these problems it is important to understand the concept.
When interpreting blood gas results the following heuristic is useful:
- a base excess > +3 = metabolic alkalosis
- a base excess < -3 = metabolic acidosis