Calculation of the base excess or deficit is a way of quantifying HCO3-.
Base excess is the quantity of base (HCO3-, 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 PCO2 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, PaCO2 is 28, and the Hb is 15 gm/dl
a) Find the difference between PaCO2 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
c1) If the PaCO2 is > 40 mm Hg, subtract half the difference from 7.40 to estimate a predicted respiratory pH
c2) If the PaCO2 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.