Blood gases

  • Definition
    • Blood gases are a measurement of how much oxygen and carbon dioxide are in your blood. They also determine the acidity (pH) of your blood.

  • Alternative Names
    • Arterial blood gas analysis; ABG

  • How the Test is Performed
    • Usually, blood is taken from an artery. In some cases, blood from a vein may be used.

      Blood may be collected from one of the following arteries:

      • Radial artery in the wrist
      • Femoral artery in the groin
      • Brachial artery in the arm

      The health care provider may test circulation to the hand before taking a sample of blood from the wrist area.

      The health care provider will insert a small needle through the skin into the artery. The sample is quickly sent to a laboratory for analysis to ensure accurate results.

  • How to Prepare for the Test
    • There is no special preparation. If you are on oxygen therapy, the oxygen concentration must remain constant for 20 minutes before the test.

      Tell your health care provider if you are taking any blood-thinning medicines (anticoagulants), including aspirin.

  • How the Test will Feel
    • When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. This soon goes away.

  • Why the Test is Performed
    • The test is used to evaluate respiratory diseases and conditions that affect the lungs. It helps determine the effectiveness of oxygen therapy. The test also provides information about the body's acid/base balance, which can reveal important clues about lung and kidney function and the body's general metabolic state.

  • Normal Results
    • Values at sea level:

      • Partial pressure of oxygen (PaO2): 75 - 100 mmHg
      • Partial pressure of carbon dioxide (PaCO2): 38 - 42 mmHg
      • Arterial blood pH: 7.38 - 7.42
      • Oxygen saturation (SaO2): 94 - 100%
      • Bicarbonate - (HCO3): 22 - 28 mEq/L

      Note: mEq/L = milliequivalents per liter; mmHg = millimeters of mercury

      At altitudes of 3,000 feet and above, the oxygen value is lower.

      Normal value ranges may vary slightly among different laboratories. Some laboratories use different measurements or may test different specimens. Talk to your doctor about the meaning of your specific test results.

  • What Abnormal Results Mean
    • Abnormal results may be due to lung, kidney, or metabolic diseases. Head or neck injuries or other injuries that affect breathing can also lead to abnormal results.

  • Risks
    • There is very little risk when the procedure is done correctly. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

      Other risks associated with this test may include:

      • Bleeding at the puncture site
      • Blood flow problems at puncture site (rare)
      • Bruising at the puncture site
      • Delayed bleeding at the puncture site
      • Fainting or feeling light-headed
      • Hematoma (blood accumulating under the skin)
      • Infection (a slight risk any time the skin is broken)
  • References
    • Effros RM, Swenson ER. Acid-base balance. In: Mason RJ, Broaddus CV, Martin TR, et al. Murray & Nadel's Textbook of Respiratory Medicine. 5th ed. Philadelphia, PA: Saunders Elsevier; 2010:chap 7.

      Mayer AS, Maier LA. Evaluation of respiratory impairment and disability. In: Mason RJ, Murray JF, Broaddus VC, et al., eds. Murray and Nadel's Textbook of Respiratory Medicine. 5th ed. Philadelphia, PA: Saunders Elsevier; 2010:chap 27.