Base Excess: An Important Concept in the Study of Acid-Base

Acid-base physiology involves various interplays of ions and molecules that affect the pH of the body’s internal environment. One important concept in the study of acid-base disorders is base excess. In cases of acid-base disorders of mixed origins, measurement of the respiratory and metabolic components alone may be insufficient to identify underlying conditions, thus making determination of base excess necessary.

Base Excess definition

In 1958, a year after the standard bicarbonate was introduced, Poul Astrup and Ole Siggaard-Andersen introduced base excess as a better method to calculate the metabolic component. Base excess is simply the amount of acid or alkali needed to bring back the plasma’s normal pH under standard conditions, in vitro (in laboratory conditions and not in human subjects).

Standard Base Excess definition

The human body naturally buffers plasma and the extracellular fluid, preventing overly fluctuating pH levels of the blood. This behavior is similar to the behavior of anemic blood, and the calculation used to derive the standard base excess is based on this property. This measurement is used to predict the amount of acid or alkali required to normalize pH under standard conditions, in-vivo (in actual human subjects).


Base excess is reported in units of mEq/L, and the normal value can range from -2 to +2. A base excess value of +4 means that the body contains 4 equivalents more of base than acid, and the condition is therefore alkalosis. This means that 4 equivalents of acid are needed in order to return the pH to normal values. On the other hand, a base excess value of -4 means than the body has a negative base excess (base deficit) and is lacking 4 equivalents of base, and therefore requires more base to be added in order to normalize the pH value.

Acid-base physiology may sometimes appear complex and difficult to understand. However, an organized approach greatly aids understanding of its basic concepts. offers an interactive and easily comprehensible course covering a wide range of acid-base topics, which is designed for health-care professionals of all levels.