Complete Blood Count (CBC)
Why It Is Done
complete blood count may be done to:
- Find the cause of symptoms such as fatigue,
weakness, fever, bruising, or weight loss.
- See how much blood has been lost if there is
- Diagnose polycythemia.
- Find an
- Diagnose diseases of the blood, such as
- Check how the body is dealing with some types of drug or
- Check how abnormal bleeding is affecting the
blood cells and counts.
- Screen for high and low values before a surgery.
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
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
- 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
- Put the needle into the vein. More than one needle stick
may be needed.
- Attach a tube to the needle to fill it with
- Remove the band from your arm when enough blood is
- Put a gauze pad or cotton ball over the needle site as
the needle is removed.
- Put pressure to the site and then a
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
There is very little chance of a problem from
having a blood sample taken from a vein.
- You may get a small bruise at the site. You
can lower the chance of bruising by keeping pressure on the site for several
- In rare cases, the vein may become swollen after the
blood sample is taken. This problem is called phlebitis. A warm compress can be
used several times a day to treat this.
- 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.