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Blood Test: Complete Blood Count
What Is a Blood Test?
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 detect problems like a disease or medical condition. Sometimes, blood tests can help them see how well an organ (such as the liver or kidneys) is working.
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 a child 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, infections, inflammation, and other conditions.
How Should We Prepare for a CBC?
Your child should be able to eat and drink normally unless also getting other tests that require fasting beforehand. Tell your doctor about any medicines your child takes because some drugs might affect the test results. Also let the doctor know if your child has had a blood transfusion or smokes. These can affect the CBC.
Wearing a T-shirt or short-sleeved shirt for the test can make things easier for your child, and you also can bring along a toy or book as a distraction.
How Is a CBC Done?
Most blood tests take a small amount of blood from a vein. To do that, a health professional will:
- clean the skin
- put an elastic band (tourniquet) above the area to get the veins to swell with blood
- 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
In babies, blood draws are sometimes done as a "heel stick collection." After cleaning the area, the health professional will prick your baby's heel with a tiny needle (or lancet) to collect a small sample of blood.
Collecting a sample of blood is only temporarily uncomfortable and can feel like a quick pinprick.
Can I Stay With My Child During a CBC?
Parents usually can stay with their child during a blood test. Encourage your child to relax and stay still because tensing muscles can make it harder to draw blood. Your child might want to look away when the needle is inserted and the blood is collected. Help your child to relax by taking slow deep breaths or singing a favorite song.
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 it.
Are There Any Risks From CBCs?
A CBC is a safe procedure with minimal risks. Some kids might feel faint or lightheaded from the test. A few kids and teens have a strong fear of needles. If your child is anxious, talk with the doctor before the test about ways to make the procedure easier.
A small bruise or mild soreness around the blood test site is common and can last for a few days. Get medical care for your child 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.
Reviewed by: Larissa Hirsch, MD
Date reviewed: October 18, 2017