Calculation of the base excess or deficit is a way of quantifying HCO_{3}^{-}.

Base excess is the quantity of base (HCO_{3}^{-}, in mEq/L) that is above or below the normal range of buffer base in the body (22 -28 mEq/L). This cannot be calculated from PCO_{2} and pH as the hemoglobin also contributes to the buffer base. One can use the Siggaard-Andersen nomogram to estimate base excess or deficit. Another way is to estimate using the following calculations:

For example: if the pH is 7.24, PaCO_{2} is 28, and the Hb is 15 gm/dl

a) Find the difference between PaCO_{2} and 40 mm Hg^{}

40 mm Hg - 28 mm Hg = 12 mm Hg

b) Move the decimal two places to the left

12 becomes 0.12

c_{1}) If the PaCO_{2} is > 40 mm Hg, subtract half the difference from 7.40 to estimate a predicted respiratory pH

c_{2}) If the PaCO_{2} is < 40 mm Hg, add the entire difference to 7.40.

7.40 + 0.12 = 7.52

d) Find the difference between the measured and predicted respiratory pH

7.24 - 7.52 = - 0.28

e) Move the decimal two places to the right

-0.28 becomes -28

f) Multiply by 2/3 to calculate base excess

(-28 x 2)/3 = -18 mEq/L

Base excess/deficit of +/- 2 mEq/L is normal.

Severe metabolic acidosis is associated with a base deficit of -10 mEq/L

A positive number is called a base excess and indicates a metabolic alkalosis.

A negative number is called a base deficit and indicates a metabolic acidosis.