Carbon Dioxide, Total

Serum (preferred) or plasma

1 mL

0.7 mL (Note: This volume does not allow for repeat testing.)

Red-top tube, gel-barrier tube, or green-top (lithium heparin) tube. Do not use oxalate, EDTA, or citrate plasma.

Evaluate the total carbonate buffering system in the body, acid-base balance. High results may represent respiratory acidosis with CO₂ retention, or metabolic alkalosis (eg, prolonged vomiting). Low value may indicate respiratory alkalosis as in hyperventilation or metabolic acidosis (eg, diabetes with ketoacidosis).

Interpretation requires clinical information and the other electrolytes.

Enzymatic

See table.

“Total carbon dioxide” consists of CO₂ in solution or bound to proteins, HCO₃⁻, CO₃²⁻, and H₂CO₃. In practice, 80% to 90% is present as bicarbonate (HCO₃⁻). “Hypercapnia” means excessive carbon dioxide in the blood. Impaired elimination of CO₂ reflects interaction of abnormalities in respiratory drive, the muscles of respiration, and the function of the lung. Elimination of carbon dioxide from the lung involves alveolar ventilation but not dead-space ventilation. Partitioning of these spaces is expressed as a ratio between dead space and total volume per breath: the tidal volume. The tidal volume normally is <0.30. These and other aspects of pulmonary gas exchange, ventilation and their consequences are addressed as the partial pressure of arterial carbon dioxide, PaCO₂, a part of arterial blood gases.¹

Note: Total CO₂ (Bicarbonate) results should be interpreted with caution, because escape of dissolved CO₂ from the sample prior to analysis is inevitable and will vary among laboratory locations. This is because the measured total CO₂ in serum decreases relative to the amount of time that the sample is open to the atmosphere. CO₂ loss occurs the moment the stopper is removed from the vacutainer tube. Labcorp facilities follow rigorous quality processes in order to reduce the exposure of patient samples to the atmosphere and minimize CO₂ loss, but even minor logistical variations at our laboratory locations may cause differences in the degree of CO₂ loss.