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Home :: Total Carbon Dioxide Content

Total Carbon Dioxide Content

When 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.


  • To help evaluate acid-base balance.

Patient preparation

  • Explain to the patient that this test is performed to measure the amount of CO2 in the blood.
  • Tell him that the test requires a blood sample. Explain who will perform the venipuncture and when.
  • Explain that he may experience discomfort from the needle puncture and the tourniquet but that collecting the sample usually takes less than 3 minutes.
  • Inform the patient that he needn't restrict food or fluids before the test.
  • Check the patient's history for use of medications that may affect test results, including corticotropin, cortisone, thiazide diuretics, salicylates, paraldehyde, methicillin, dimercaprol, ammonium chloride, and acetazolamide.
  • Also note excessive ingestion of alkalis or licorice or accidental ingestion of ethylene glycol or methyl alcohol.

Procedure and posttest care

  • Perform a venipuncture.
  • When CO2 content is measured along with electrolytes, a 7-ml red­ marble-top tube may be used.
  • When this test is performed alone, a green-top (heparinized) tube is appropriate.
  • If a hematoma develops at the venipuncture site, apply warm soaks
  • Fill the tube completely to prevent diffusion of CO2 into the vacuum

Reference values

Normally, total CO2 levels range from 23 to 30 mEq/L. Levels may vary, depending on sex and age.
Abnormal findings

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.

Interfering factors

  • Excessive use of corticotropin, cortisone, or thiazide diuretics; excessive ingestion of alkalis or licorice (increase)
  • Salicylates, paraldehyde, methicillin, dimercaprol, ammonium chloride, and acetazolamide; ingestion of ethylene glycol or methyl alcohol (decrease).

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Disclaimer: website is designed for educational purposes only. It is not intended to treat, diagnose, cure, or prevent any disease. Always take the advice of professional health care for specific medical advice, diagnoses, and treatment. We will not be liable for any complications, or other medical accidents arising from the use of any information on this web site. Please note that medical information is constantly changing. Therefore some information may be out of date.