What does total CO2 measure approximately?

Total CO2 measures the concentration of bicarbonate (HCO3-) in the blood, which is a critical parameter in assessing acid-base balance and respiratory function. The total CO2 content reflects the bicarbonate ions present as well as dissolved carbon dioxide and carbonic acid in the serum. When evaluating metabolic and respiratory states, interpreting bicarbonate levels is vital because they indicate how well the body is managing acid-base status. This measurement is essential in diagnosing and monitoring conditions such as respiratory acidosis, metabolic acidosis, and other disorders affecting the body's pH balance.

The other options correspond to different substances in the blood that are distinctly measured through different tests. Therefore, understanding that total CO2 predominantly represents bicarbonate helps clarify its role in the broader context of laboratory medicine and patient care.