The total CO2 test measures the total amount of carbon dioxide in the blood, which occurs mostly in the form of bicarbonate (HCO3-). Bicarbonate is a negatively charged ion that is excreted and reabsorbed by the kidneys. It is used by the body to help maintain the body's acid-base balance (pH) and secondarily to work with sodium, potassium, and chloride to maintain electrical neutrality at the cellular level.