Carbon Dioxide, Total (CO2)

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Turnaround Time: Within 1 day
CPT Code:


Test Type: 1 mL Serum (preferred) OR plasma
Stability Time:



Room temperature

14 days


14 days


14 days

Freeze/thaw cycles

Stable x3

Reference Range:


Range (mmol/L)

0 to 30 d


31 d to 1 y


2 to 5 y


6 to 12 y


>12 y



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.

“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

1. Weinberger SE, Schwartzstein RM, Weiss JW. Hypercapnia. N Engl J Med. 1989 Nov 2; 321(18):1223-1231.PubMed2677729

Collection Details:

Collection Instructions:

Maintain specimen in a tightly-stoppered tube.

Red-top tube, gel-barrier tube, or green-top (lithium heparin) tube. Do NOT use Oxalate, EDTA, or Citrate Plasma.

Draw blood in gel-barrier tube and centrifuge. If red-top tube or green-top tube is used, centrifuge within 45 minutes of draw, remove the serum or plasma, and place in a transport tube and tightly stopper the tube.

Room temperature.