Total Carbon Dioxide ContentWhen carbon dioxide (CO2) pressure in red blood cells exceeds 40 mm Hg, CO2 spills out of the cells and dissolves in plasma. There it may combine with water to form carbonic acid, which in turn may dissociate into hydrogen and bicarbonate ions. This test is used to measure the total concentration of all forms of CO2 in serum, plasma, or whole blood samples. It's commonly ordered for patients with respiratory insufficiency and is usually included in an assessment of electrolyte balance. Test results are most significant when considered with pH and arterial blood gas values.
Procedure and posttest care
Reference valuesNormally, total CO2 levels range from 23 to 30 mEq/L. Levels may vary, depending on sex and age.
High CO2 levels may occur in metabolic alkalosis, respiratory acidosis, primary aldosteronism, and Cushing's syndrome. CO2 levels may also increase after excessive loss of acids, as in severe vomiting and continuous gastric drainage.
Decreased CO2 levels are common in metabolic acidosis. Decreased total CO2 levels in metabolic acidosis also result from loss of bicarbonate. Levels may decrease in respiratory alkalosis.
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