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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 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) 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 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 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

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 favorite music.

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 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 easier.

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.

Reviewed by: Larissa Hirsch, MD
Date Reviewed: 23-01-2018