Test Overview

A bicarbonate (HCO3) test measures the level of bicarbonate in the blood. Bicarbonate is a chemical (buffer) that keeps the pH of blood from becoming too acidic or too basic.

Bicarbonate is not usually tested by itself. It may be done on a blood sample taken from a vein as part of a panel of tests that looks at other electrolytes, such as sodium, potassium, and chloride. It is often done along with a carbon dioxide test or as part of an arterial blood gas (ABG) test. For an arterial blood gas study, the blood sample is taken from an artery.

Why It Is Done

A bicarbonate (HCO3) test helps find and keeps track of conditions that affect blood bicarbonate levels, including many kidney diseases, some lung diseases, and metabolic conditions.

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 using oxygen, it may be turned off for 20 minutes before an arterial blood gas test. This is called a "room air" test. If you cannot breathe without the oxygen, it 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

Blood drawn from an artery

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 at least 5 to 10 minutes (possibly longer if you take blood-thinning medicine or have bleeding problems).

Blood drawn from a vein

The health professional drawing blood will:

• Wrap an elastic band around your arm to stop the flow of blood.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Apply a gauze pad or cotton ball over the needle site as the needle is removed.
• Apply pressure to the site and then a bandage.

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, if your artery is narrowed, or if you are very sensitive to pain.

If the blood sample is taken from a vein in your arm, an elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

Risks

Risks of blood drawn from an artery

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.

Risks of a blood drawn from a vein

There is very little risk of complications from having blood drawn from a vein.

• You may develop a small bruise at the puncture site. You can reduce the risk of bruising by keeping pressure on the site for several minutes after the needle is withdrawn.
• In rare cases, the vein may become inflamed after the blood sample is taken. This condition is called phlebitis and is usually treated with a warm compress applied several times daily.
• Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can also make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your health professional before your blood is drawn.

Results

A bicarbonate (HCO3) test measures the level of bicarbonate in the blood.

Normal values may vary from lab to lab. Results are usually available right away.

Bicarbonate

Normal:	20–29 mEq/L (20–29 mmol/L)

High values

High HCO3 levels may be caused by:

• Vomiting, dehydration, blood transfusions, and overuse of medicines that contain bicarbonate (especially antacids).
• Conditions such as chronic obstructive pulmonary disease (COPD), fluid in the lungs (pulmonary edema), heart disease, Cushing's disease, and Conn's syndrome.
• Medicines, such as water pills (diuretics) and corticosteroids, such as hydrocortisone and prednisone.

Low values

Low HCO3 levels may be caused by:

• Hyperventilation , aspirin or alcohol overdose, diarrhea, dehydration, severe malnutrition, severe burns, and shock.
• Liver or kidney disease, a massive heart attack, hyperthyroidism, or uncontrolled diabetes.
• Certain medicines, such as medicine used to treat glaucoma and some antibiotics.

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 drink liquids that are very acidic, such as orange juice or some types of carbonated beverages, shortly before having the test.
• The health professional leaves on the elastic band that stops blood flow for an extended amount of time before the blood sample is collected.

What To Think About

• The blood bicarbonate test is often done along with an arterial blood gas (ABG) test. An ABG test measures the blood pH and carbon dioxide levels that are needed to find the cause of a high or low bicarbonate level. For more information, see the medical test Arterial Blood Gases.
• The blood bicarbonate test is often done along with the test for carbon dioxide. For more information, see the medical test Carbon Dioxide.

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.

Credits

Author	Maria G. Essig, MS, ELS
Editor	Susan Van Houten, RN, BSN, MBA
Associate Editor	Tracy Landauer
Primary Medical Reviewer	Anne C. Poinier, MD - Internal Medicine
Specialist Medical Reviewer	Robert L. Cowie, MB, FCP(SA), MD, MSc, MFOM - Pulmonology
Last Updated	June 17, 2008