Arterial Blood Gases

Test Overview

An arterial blood gas (ABG) test measures the acidity (pH) and the levels of oxygen and carbon dioxide in the blood from an artery. This test is used to check how well your lungs are able to move oxygen into the blood and remove carbon dioxide from the blood.

As blood passes through your lungs, oxygen moves into the blood while carbon dioxide moves out of the blood into the lungs. An ABG test uses blood drawn from an artery, where the oxygen and carbon dioxide levels can be measured before they enter body tissues. An ABG measures:

• Partial pressure of oxygen (PaO2). This measures the pressure of oxygen dissolved in the blood and how well oxygen is able to move from the airspace of the lungs into the blood.
• Partial pressure of carbon dioxide (PaCO2). This measures how much carbon dioxide is dissolved in the blood and how well carbon dioxide is able to move out of the body.
• pH. The pH measures hydrogen ions (H+) in blood. The pH of blood is usually between 7.35 and 7.45. A pH of less than 7.0 is called acid and a pH greater than 7.0 is called basic (alkaline). So blood is slightly basic.
• Bicarbonate (HCO3). Bicarbonate is a chemical (buffer) that keeps the pH of blood from becoming too acidic or too basic.
• Oxygen content (O2CT) and oxygen saturation (O2Sat) values. O2 content measures the amount of oxygen in the blood. Oxygen saturation measures how much of the hemoglobin in the red blood cells is carrying oxygen (O2).

Blood for an ABG test is taken from an artery. Most other blood tests are done on a sample of blood taken from a vein, after the blood has already passed through the body's tissues where the oxygen is used up and carbon dioxide is produced.

Why It Is Done

An arterial blood gas (ABG) test is done to:

• Check for severe breathing problems and lung diseases, such as asthma, cystic fibrosis, or chronic obstructive pulmonary disease (COPD).
• See how well treatment for lung diseases is working.
• Find out if you need extra oxygen or help with breathing (mechanical ventilation).
• Find out if you are receiving the right amount of oxygen when you are using oxygen in the hospital.
• Measure the acid-base level in the blood of people who have heart failure, kidney failure, uncontrolled diabetes, sleep disorders, severe infections, or after a drug overdose.

How To Prepare

Tell your doctor if you:

• Have had bleeding problems or take blood thinners, such as aspirin or warfarin (Coumadin).
• Are taking any medicines.
• Are allergic to any medicines, such as those used to numb the skin (anesthetics).

If you are on oxygen therapy, the oxygen may be turned off for 20 minutes before the blood test. This is called a "room air" test. If you cannot breathe without the oxygen, the oxygen will not be turned off.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results may mean. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?) .

How It Is Done

A sample of blood from an artery is usually taken from the inside of the wrist (radial artery), but it can also be collected from an artery in the groin (femoral artery) or on the inside of the arm above the elbow crease (brachial artery). You will be seated with your arm extended and your wrist resting on a small pillow. The health professional drawing the blood may rotate your hand back and forth and feel for a pulse in your wrist.

To prevent the possibility of damaging the artery of the wrist when the blood sample is taken, a procedure called the Allen test may be done to ensure that blood flow to your hand is normal. An arterial blood gas (ABG) test will not be done on an arm used for dialysis or if there is an infection or inflammation in the area of the puncture site.

The health professional taking a sample of your blood will:

• Clean the needle site with alcohol. You may be given an injection of local anesthetic to numb that area.
• Put the needle into the artery. More than one needle stick may be needed.
• Allow the blood to fill the syringe. Be sure to breathe normally while your blood is being collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put a bandage over the puncture site and apply firm pressure for 5 to 10 minutes (possibly longer if you take blood-thinning medicine or have bleeding problems).

How It Feels

Collecting blood from an artery is more painful than collecting it from a vein because the arteries are deeper and are protected by nerves.

• Most people feel a brief, sharp pain as the needle to collect the blood sample enters the artery. If you are given a local anesthetic, you may feel nothing at all from the needle puncture, or you may feel a brief sting or pinch as the needle goes through the skin.
• You may feel more pain if the person drawing your blood has a hard time finding your artery, your artery is narrowed, or if you are very sensitive to pain.

Risks

There is little chance of a problem from having blood sample taken from an artery.

• You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for at least 10 minutes after the needle is removed (longer if you have bleeding problems or take blood thinners).
• You may feel lightheaded, faint, dizzy, or nauseated while the blood is being drawn from your artery.
• Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.
• On rare occasions, the needle may damage a nerve or the artery, causing the artery to become blocked.

Though problems are rare, be careful with the arm or leg that had the blood draw. Do not lift or carry objects for about 24 hours after you have had blood drawn from an artery.

Results

An arterial blood gas (ABG) test measures the acidity (pH) and levels of oxygen and carbon dioxide in the blood.

Normal

Normal values may vary from lab to lab and depend upon the elevation above sea level. Results are usually available right away.

The concentration of oxygen being breathed, called the fraction of inhaled oxygen (FiO2), is also usually reported. This is only useful if you are receiving oxygen therapy from a tank or are on a ventilator.

Many conditions can change blood gas levels. Your doctor will talk with you about any abnormal results that may be related to your symptoms and medical history.

Abnormal results may be caused by:

• Diabetes.
• Drug or alcohol overdose.
• Kidney failure.
• Lung diseases, such as chronic lung disease, a blood clot in the lungs, or pneumonia.
• Severe dehydration from severe burns, vomiting, diarrhea, or taking water pills (diuretics).
• Severe infection (sepsis).

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include the following:

• You have a fever or an abnormally low body temperature (hypothermia).
• You have a disease that affects how much oxygen is carried in your blood, such as severe anemia or polycythemia.
• You smoke just before the test or breathe secondhand smoke, carbon monoxide, or certain paint or varnish removers in closed or poorly ventilated areas.

What To Think About

• Arterial blood gas (ABG) values alone do not provide enough information to diagnose a problem. For example, they cannot tell whether low levels are caused by lung or heart problems. Arterial blood gas values are most helpful when they are reviewed with other examinations and tests.
• An ABG test is often done for a person who is in the hospital because of severe injury or illness. The test can measure how well the person's lungs and kidneys are working and how well the body is using energy.
• An ABG test may be most useful when a person's breathing rate is increased or decreased or when the person has very high blood sugar (glucose) levels, a severe infection, or heart failure.
• If several blood samples are needed, a thin tube (arterial catheter) may be placed in an artery. Blood can then be collected when needed.

References

Other Works Consulted

• Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.

• Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.

• Pagana KD, Pagana TJ (2006). Mosby's Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.

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