Blood gases is a measurement of how much oxygen and carbon dioxide is in your blood. It also determines the acidity (pH) of your blood.

Usually, blood is taken from an artery. The blood may be collected from the radial artery in the wrist, the femoral artery in the groin, or the 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. You can choose to have numbing medicine (anesthesia) applied to the site before the test begins.

In rare cases, blood from a vein may be used.

After the blood is taken, pressure is applied to the site for a few minutes to stop the bleeding. The health care provider will watch the site for signs of bleeding or circulation problems.

The sample must be quickly sent to a laboratory for analysis to ensure accurate results.

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

You may feel brief cramping or throbbing at the puncture site.

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.

See also:

• Acidosis
• Diabetic ketoacidosis
• Lactic acidosis
• Metabolic acidosis
• Respiratory acidosis
• Respiratory alkalosis

Values at sea level:

• Partial pressure of oxygen (PaO2) - 75 - 100 mmHg
• Partial pressure of carbon dioxide (PaCO2) - 35 - 45 mmHg
• A pH of 7.35 - 7.45
• Oxygen saturation (SaO2) - 94 - 100%
• Bicarbonate - (HCO3) - 22 - 26 mEq/L

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

At altitudes of 3,000 feet and above, the oxygen values are lower.

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

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.

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)

Tell your health care provider if you notice bleeding, bruising, numbness, tingling, or discoloration at the puncture site. Also let your doctor know if you are taking any blood-thinning medications (anticoagulants) or aspirin.

Gold WG. Pulmonary function testing. In: Mason RJ, Murray J, VC Broaddus, Nadel J, eds. Textbook of Respiratory Medicine. 3rd ed. Philadelphia, Pa: Saunders Elsevier; 2005:chap 24.