Base excess is defined as the amount of strong acid that must be added to each liter of fully oxygenated blood to return the pH to 7.40 at a temperature of 37°C and a pCO of . A base deficit (i.e., a negative base excess) can be correspondingly defined by the amount of strong base that must be added.
A further distinction can be made between actual and standard base excess: actual base excess is that present in the blood, while standard base excess is the value when the hemoglobin is at 5 g/dl. The latter gives a better view of the base excess of the entire extracellular fluid.
Base excess (or deficit) is one of several values typically reported with arterial blood gas analysis that is derived from other measured data.
The term and concept of base excess were first introduced by Poul Astrup and Ole Siggaard-Andersen in 1958.