Complete Blood Count (CBC)
Why It Is Done
A complete blood count may be done to:
- Find the cause of symptoms such as fatigue, weakness, fever, bruising, or weight loss.
- Check for anemia.
- See how much blood has been lost if there is bleeding.
- Diagnose polycythemia.
- Check for an infection.
- Diagnose diseases of the blood, such as leukemia.
- Check how the body is dealing with some types of drug or radiation treatment.
- Check how abnormal bleeding is affecting the blood cells and counts.
- Screen for high and low values before a surgery.
- See if there are too many or too few of certain types of cells. This may help find other conditions, such as too many eosinophils may mean an allergy or asthma is present.
A complete blood count may be done as part of a regular physical examination. A blood count can give valuable information about the general state of your health.
How To Prepare
You do not need to do anything before having this test.
How It Is Done
Your health professional drawing blood will:
- Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
- 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.
- Put a gauze pad or cotton ball over the needle site as the needle is removed.
- Put pressure to the site and then a bandage.
If this blood test is done on a baby, a heel stick will be done instead of a blood draw from a vein.
How It Feels
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.