Abnormality of acid-base homeostasis Abnormal base excess 2019-01-27T23:07:10Z The base excess increases in metabolic alkalosis and decreases in metabolic acidosis, but its utility in interpreting blood gas results is controversial. The term base excess is used in clinical practice, but the available blood gas devices calculate either the standard base excess, also called the base excess of the extracellular fluid, or the base excess of blood. Base deficit is calculated as the base excess times minus one. Deviation from the normal quantity of base excess, defined as the amount of strong acid (in millimoles per liter) that needs to be added in vitro to 1 liter of fully oxygenated whole blood to return a blood sample to standard conditions (pH of 7.40, Pco2 of 40 mm Hg, and temperature of 37 degrees C).