The value is usually reported as a concentration in units of mEq/L (mmol/L), with positive numbers indicating an excess of base and negative a deficit.
[1] Comparison of the base excess with the reference range assists in determining whether an acid/base disturbance is caused by a respiratory, metabolic, or mixed metabolic/respiratory problem.
However, base excess is a more comprehensive measurement, encompassing all metabolic contributions.
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 pCO2 of 40 mmHg (5.3 kPa).
[3] Base excess (or deficit) is one of several values typically reported with arterial blood gas analysis that is derived from other measured data.
[2] The term and concept of base excess were first introduced by Poul Astrup and Ole Siggaard-Andersen in 1958.
A secondary (compensatory) process can be readily identified because it opposes the observed deviation in blood pH.
The kidneys only partially compensate, so the patient may still have a low blood pH, i.e. acidemia.
In summary, the kidneys partially compensate for respiratory acidosis by raising blood bicarbonate.