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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 levels 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?

Doctors may order a CBC as part of a routine checkup to screen for problems or because a person isn't feeling well. The levels of red blood cells, white blood cells, and platelets can help doctors find possible problems like anemia, 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.

What if I Have Questions?

If you have questions about the CBC or what the test results mean, talk to your doctor. 

Reviewed by: Larissa Hirsch, MD
Date Reviewed: Mar 20, 2023

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