Standard bicarbonate is the concentration of bicarbonate in the plasma from blood which is equilibrated with a normal PaCO2 (40 mmHg) and a normal pO2 (over 100 mmHg) at a normal temperature (37°C). Usually, it is obtained by solving the Henderson-Hasselbalch equation to get a bicarbonate value when the pH is known and PaCO2 is 40mmHg
This bicarbonate level represents the metabolic contribution to the change in bicarbonate. It was introduced in 1957 by Jorgensen and Astrup. In essence, this is what the bicarbonate should be if all the non-metabolic influences were corrected. It answer the question, "how much would my patient's bicarbonate be if I were ventilating them properly?".
The equation for standard bicarbonate incorporates the actual base excess value.
The utility of the standard bicarbonate stems from its rejection of the temperature and of the normal influence on buffering which results from changes in oxygen and carbon dioxide concentration. With these variables out of the picture, the only remaining influences on bicarbonate are metabolic. The standard bicarbonate can therefore be said to represent the buffering capacity of the blood more accurately than the actual bicarbonate. Obviously, in states of vigorous health the actual bicarbonate and the standard bicarbonate are more or less identical. However, the greater the respiratory acid-base disturbance, the greater the difference between these two values.