A blood test is
when a sample of blood is taken
from the body to be tested in a lab. Doctors order blood tests to check things such
as the levels of glucose,
hemoglobin, or white blood cells. This can help them find problems like a disease
or medical condition. Sometimes, blood tests can help them see how well an organ (such
as the liver or kidneys)
What Is a Complete Blood Count?
A complete blood count (CBC) test is a blood test that helps doctors check the
level of different types of cells in the blood. A CBC measures:
red blood cells, which deliver oxygen to the different parts of the body
white blood cells, which help fight infections
platelets, which help blood clot to stop bleeding
Why Are CBCs Done?
A CBC can be done as part of a routine checkup to screen for problems or because
someone isn't feeling well. The levels of red blood cells, white blood cells, and
platelets can provide doctors with information about possible problems like anemia
(low red blood cells), infections, inflammation, and other conditions.
How Should I Prepare for a CBC?
You should be able to eat and drink normally unless also getting other tests that
require fasting beforehand. Tell your doctor about any medicines you take because
some drugs might affect the test results. Also let the doctor know if you have had
a blood transfusion
or if you smoke. These can also affect the CBC.
It can help to wear a T shirt or other short-sleeve top on the day of the test
to make things faster and easier for the technician who will be drawing the blood.
How Is a CBC Done?
Most blood tests take a small amount of blood from a vein. To do that, a health
clean the skin
put an elastic band (tourniquet) above the area to get the veins to swell with
insert a needle into a vein (usually in the arm inside of the elbow or on the
back of the hand)
pull the blood sample into a vial or syringe
take off the elastic band and remove the needle from the vein
It's best to try to relax and stay still during the procedure because tensing muscles
can make it harder and more painful to draw blood. And if you don't want to watch
the needle being inserted or see the blood collecting, you don't have to. Look the
other way and maybe relax by focusing on saying the alphabet backward, doing some
breathing exercises, thinking of a place that makes you happy, or listening to your
How Long Does a CBC Take?
Most blood tests take just a few minutes. Occasionally, it can be hard to find
a vein, so the health professional may need to try more than once.
What Happens After a CBC?
The health professional will remove the elastic band and the needle and cover the
area with cotton or a bandage to stop the bleeding. Afterward, there may be some mild
bruising, which should go away in a few days.
When Are CBC Results Ready?
Blood samples are processed by a machine, and it may take a few hours to a day
for the results to be available. If the test results show signs of a problem, the
doctor might order other tests to figure out what the problem is and how to treat
Are There Any Risks From CBCs?
A CBC is a safe procedure with minimal risks. Some people might feel faint or lightheaded
from the test. A few teens have a strong fear of needles.
If you're anxious, talk with the doctor before the test about ways to make the procedure
A small bruise or mild soreness around the blood test site is common and can last
for a few days. Get medical care if the discomfort gets worse or lasts longer.
If you have questions about the CBC, speak with your doctor or the health professional
doing the blood draw.