Overview & Description. This test is used to determine the amount of bicarbonate, called HCO3, in the blood. The test actually measures the blood level of carbon dioxide, called CO2. During laboratory testing, HCO3 is converted to CO2. HCO3 is important in neutralizing acids.he normal value for CO2 in the blood is 23 to 29 mEq/L. Most of the carbon dioxide in the blood is in the form of bicarbonate. Either term can be used.