Blood gases is a test done to measure how much oxygen and carbon dioxide is in your blood. It also looks at the acidity (pH) of the blood. Usually, blood gases look at blood from an artery. In rarer cases, blood from a vein may be used.

The test is performed using a small needle to collect a sample of blood from an artery. The sample may be collected from the radial artery in the wrist, the femoral artery in the groin, or the brachial artery in the arm.

Before blood is drawn, the health care provider may test circulation to the hand (if the wrist is the site). After the blood is drawn, pressure applied to the puncture site for a few minutes stops the bleeding.

The test must be sent to the laboratory for analysis quickly to ensure accurate results.

There is no special preparation. If the person having the test is on oxygen, the oxygen concentration must remain constant for 20 minutes before the test. If the test is done without oxygen, the oxygen must be turned off for 20 minutes before the sample is taken to ensure accurate test results.

The health care provider will insert a needle through the skin into the artery. You can choose to have anesthesia at the site. You may feel brief cramping or throbbing at the puncture site. The needle will be withdrawn after the sample is collected.

Pressure applied over the site for 5 - 10 minutes helps prevent bleeding. A bandage will be placed over the puncture site. The health care provider will watch the site for signs of bleeding or circulation problems.

The test is used to evaluate respiratory diseases and conditions that affect the lungs. It helps determine the effectiveness of oxygen therapy. The acid-base component of the test also gives information about kidney function.

Values at sea level:

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

Note: mEq/liter = milliequivalents per liter; mm Hg = millimeters of mercury

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

Abnormal results may indicate respiratory, metabolic, or renal diseases. The results may also be abnormal with head or neck injuries, or other traumas that affect breathing.

In general, there is a very low risk when the procedure is done correctly. There may be bleeding or bruising at the puncture site, or delayed bleeding from the site. Circulatory impairment in the area of the puncture may occur, although it is rare.

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.

Ford M, Delaney KA, Ling L, Erickson T. Clinical Toxicology. 1st ed. Philadelphia, Pa:Saunders, 2000.

Mason RJ, Broaddus VC, Murray JF, Nadel JA. Murray and Nadel's Textbook of Respiratory Medicine. 4th ed. Philadelphia, Pa:Saunders, 2005.