A bicarbonate (HCO3) test measures the level of bicarbonate in the blood. Bicarbonate is a chemical that keeps the pH of blood from becoming too acid. If the pH level drops, HCO3 is absorbed by the kidneys and returned to the blood instead of passing out of the body in the urine.

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 calcium. 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.
• Attach a tube to the needle to fill it with blood. 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 medication 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.