What Is a Complete Blood Count?

A complete blood count (CBC) is a test that measures the cells that make up your blood: red blood cells, white blood cells, and platelets. Your doctor may order a CBC as part of a routine check-up or to:

  • Check for anemia 
  • See if you have another health issue or to explain symptoms like weakness, fever, bruising, or feeling tired
  • Keep an eye on a blood condition you already have
  • See how medications or treatments like chemotherapy are affecting your blood

If the CBC is the only blood test you’re getting that day, you can eat or drink like you normally would.

How Is a CBC Done?

It’s pretty simple and takes just a few minutes. A nurse or lab tech will take a sample of blood by inserting a needle into a vein in your arm. She’ll send it to the lab for review. You can leave and get right back to your normal routine.

What Does It Measure?

The test can tell your doctor a lot about your overall health. It measures the following things:

  • White blood cells (WBCs). These help to fight infections. If you have high WBC levels, it tells your doctor you have inflammation or infection somewhere in your body. If it’s low, you could be at risk for infection. The normal range is 4,500 to 10,000 cells per microliter (cells/mcL). (A microliter is a very tiny amount – one millionth of a liter).
  • RBC (red blood cell count). This is the number of red blood cells you have. These are important because they deliver oxygen through your body. They also help carry carbon dioxide. They also help carry carIf your RBC count is too low, you may have anemia or another condition. (If you have anemia, your blood has fewer red blood cells than normal.) The normal range for men is 4.5 million to 5.9 million cells/mcL; for women it’s 4.1 million to 5.1 million cells/mcL.
  • Hb or Hgb (hemoglobin). This is the protein in your blood that holds the oxygen. The normal range for men is 14 to 17.5 grams per deciliter (gm/dL); for women it’s 12.3 to 15.3 gm/dL.
  • Hct (hematocrit). This value provides information about how much of your blood is comprised of red blood cells. A low score on the range scale may be a sign that you have too little iron, the mineral that helps produce red blood cells. A high score could mean you’re dehydrated or have another condition. The normal range for men is between 41.5% and 50.4%. For women the range is between 36.9% and 44.6%.
  • MCV (mean corpuscular volume). This is the average size of your red blood cells. If they’re bigger than normal, your MCV goes up. That could happen if you have low vitamin B12 or folate levels. If your red blood cells are smaller, you could have a type of anemia. A normal-range MCV score is 80 to 96.
  • Platelets. These play a role in clotting. This test measures the number of platelets in your blood. The normal range is 150,000 to 450,000 platelets/mcL

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What Do My Results Mean?

When you get your report, you’ll notice two columns: one called a “reference range” and another for your results. If your results are inside the reference range, they’re normal. If your results are higher or lower than the reference range, they’re abnormal. Mild anemia is the most common reason your results might be off.

Each lab has its own special equipment and different ways of analyzing your blood. So the reference range -- what’s considered normal levels -- will depend on the lab that handles your blood tests.

It’ll also depend on you age, sex, and how high above sea level you live.

What Else Might My CBC Tell Me?

It can reveal more information about your health, depending on what your doctor orders. Your doctor will know whether you have an illness or blood condition by measuring the following:

  • MCH (mean corpuscular hemoglobin). How much hemoglobin (a protein) is in your typical red blood cell. It carries oxygen to your organs and tissues.
  • MCHC (mean corpuscular hemoglobin concentration). This measures the concentration of hemoglobin in a certain amount of blood. It is calculated by dividing the Hgb by Hct.
  • RDW (red cell distribution width). How your much your red blood cells vary in size.
  • Reticulocyte Count. This test measures the number of new red blood cells in your body.
  • MPV (mean platelet volume). The average size of the platelets in your blood.
  • PDW (platelet distribution width). How much your platelets vary in size.
  • White Blood Cell Differential. There are five types of white blood cells. This test shows how many of each type you have: neutrophils, lymphocytes, monocytes, eosinophils, and basophils.
WebMD Medical Reference Reviewed by Louise Chang, MD on December 22, 2018

Sources

SOURCES:

Mayo Clinic: “Complete Blood Count (CBC).”

American Association for Clinical Chemistry: Lab Tests Online “Complete Blood Count (CBC)

University of Rochester Medical Center: “Complete Blood Count.”

National Heart, Lung, and Blood Institute: “What Do Blood Tests Show?” “What Is Anemia?”

UCLA Health: “Complete Blood Count.”

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