According to the acid-base tutorial from Tulane University Department of Anesthesia, standard bicarbonate is the bicarbonate concentration under standard conditions of 40 mmHg pCO2, temperature of 37 degrees Celsius and saturated with oxygen. The term standard bicarbonate was introduced by Jorgensen and Astrup in 1957.
The actual bicarbonate is the concentration of bicarbonate in the blood. In acid-base measurements, the bicarbonate concentration is not measured but is calculated from the pH and the pCO2 using the Henderson-Hasselbach equation. The pH is the natural logarithm of the hydrogen ion concentration. The pCO2 is the partial pressure of carbon dioxide. The Henderson-Hasselbach equation is pH = pK + log [bicarbonate]/[carbon dioxide].
The concentration of bicarbonate reflects the acidity or alkalinity of the blood. In metabolic acidosis, the bicarbonate concentration is low, and in metabolic alkalosis the bicarbonate concentration is high. The actual bicarbonate concentration reflects not only the metabolic component but also the respiratory component. By controlling for the respiratory component, the standard bicarbonate is a better measure of the metabolic component than the actual bicarbonate.
The standard bicarbonate is the inverse of the standard pH, which is the pH under standard conditions of 40 mmHg pCO2, temperature of 37 degrees Celsius and saturated with oxygen.