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Lab Test: Carbon Dioxide, CO2 (Blood) Level

    Lab Test
    • Carbon Dioxide (CO2; Blood)
    • Total carbon dioxide content (TCO2) measurement is the sum of the bicarbonate, carbonic acid, and dissolved carbon dioxide (CO2) in plasma, serum or whole blood.
    • In the peripheral venous blood this is used to assist in evaluating the pH status of the patient and to assist in evaluation of electrolytes.
    Reference Range

    • Adults, plasma, at sea level:  21-30 mEq/L (21-30 mmol/L)
    • Adults, capillary (heparin) plasma:  22-28 mEq/L (22-28 mmol/L)
    • Adults, whole blood, arterial:  19-24 mEq/L (190-24 mmol/L)
    • Adults, whole blood, venous:  22-26 mEq/L (22-26 mmol/L)
    • Adults over 60 years, plasma or serum, venous:  23-31 mEq/L (23-31 mmol/L)
    • Adults over 90 years, plasma or serum, venous:  20-29 mEq/L (20-29 mmol/L)
    • Premature, 1 week, capillary (heparin) plasma:  14-27 mEq/L (14-27 mmol/L)
    • Newborn, capillary (heprin) plasma:  13-22 mEq/L (13-22 mmol/L)
    • Cord blood:  14-22 mEq/L (14-22 mmol/L)
    • Infant, capillary (heparin) plasma:  20-28 mEq/L (20-28 mmol/L)
    • Child, capillary (heparin) plasma:  20-28 mEq/L (20-28 mmol/L)
    • Critical values: <10 mEq/L or > 40 mEq/L

    Indications & Uses
    • Suspected metabolic acidosis - the TCO2 concentration is reduced in both metabolic acidosis and respiratory alkalosis.  A reduced blood pH measurement confirms the diagnosis of metabolic acidosis.
    • Suspected metabolic alkalosis - the TCO2 concentration is elevated in both metabolic alkalosis and respiratory acidosis.
    • Suspected respiratory acidosis - a reduced blood pH measurement confirms the diagnosis of respiratory acidosis.
    • Suspected respiratory alkalosis
    Clinical Application
    • The serum CO2 test is usually included with other electrolyte assessments.  It is important not to confuse this test with Pco2.  This CO2 content measures H2CO3, dissolved CO2 and the bicarbonate ion (HC03) that exists in the serum.  Because the amounts of H2CO3 and dissolved CO2 in the blood are so small, CO2 content is an indirect measure of HCO3 anion. 
    • Levels of HCO3 are regulated by the kidneys.  Increases occur with alkalosis, and decreases occur with acidosis. Full characterization of acid-base status requires arterial blood gas studies (i.e., pH and pCO2).  Total carbon dioxide measurement is unnecessary if arterial blood gas studies are performed. 
    • Total carbon dioxide (TCO2) may be a useful surrogate for plasma bicarbonate, because bicarbonate comprises 90% to 95% of TCO2 content.  The TCO2 overestimates the plasma bicarbonate by 1-2 mmol/L. 
    • Increased levels indicate:
      • Severe vomiting, high-volume gastric suction, aldosteronism, use of mercurial diuretics, chronic obstructive pulmonary disease (COPD), or metabolic alkalosis. 
    • Decreased levels indicate:
      • Chronic diarrhea, chronic use of loop diuretics, renal failure diabetic ketoacidosis, starvation, metabolic acidosis, or shock.
    Related Tests
    • Arterial blood gases - this is a battery of arterial blood tests that are used to evaluate acid-base status.  CO2 content and HCO3 are components of that test.
    Drug-Lab Interactions
    • Underfilling the tube with blood allows CO2 to escape from the serum specimen and may significantly reduce HCO3 values.
    • Drugs that may cause increased serum CO2 and HCO3 levels include:  aldosterone, barbiturates, bicarbonates, ethacrynic acid, hydrocortisone, loop diuretics, mercurial diuretics, and steroids.
    • Drugs that may cause decreased levels include:  methicillin, nitrofurantoin (Furadantin), paraldehyde, phenformin, tetracycline, thiazide diuretics, and triamterene.
    Test Tube Needed
    • Marble top tube* (PDR)
    • Red or green top tube
    • Collect blood sample in a heparinized syringe
    • Use the minimum amount of liquid heparin, to avoid dilutional errors and pH changes
    • Transfer blood sample anaerobically to a marbled top tube
    • Apply pressure or a pressure dressing to the venipuncture site and assess the site for bleeding.
    What To Tell Patient Before & After
    • Explain the procedure to the patient.
    • Tell the patient that no fasting is required.
    • LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests.  First ed. Montvale, NJ: Thomson PDR; 2007.
    • Pagana K, Pagana TJ eds. Mosby's Manual of Diagnostic and Laboratory Tests. 5th Ed.  St. Louis, Missouri. 2014.

MESH Terms & Keywords

  • Carbon Dioxide, CO2