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Complete Blood Count (CBC)

A lab technician tests blood samples in a blood analyzer machine.
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What is the complete blood count test (CBC)? How is it done?

The complete blood count (CBC) is one of the most commonly ordered blood tests. The complete blood count is the calculation of the cellular (formed elements) of blood. Special machines that analyze the different components of blood in less than a minute generally determine these calculations.

A major portion of the complete blood count is the measure of the concentration of white blood cells, red blood cells, and platelets in the blood.

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What are values for a complete blood count (blood cells, hematocrit, hemoglobin, and platelets)?

The values generally included:

  • White blood cell count (WBC) is the number of white blood cells in a volume of blood. Normal range varies slightly between laboratories but is generally between 4,300 and 10,800 cells per cubic millimeter (cmm). This can also be referred to as the leukocyte count and can be expressed in international units as 4.3 to 10.8 x 109 cells per liter.
  • White blood cell (WBC) differential count. White blood count is comprised of several different types that are differentiated, or distinguished, based on their size and shape. The cells in a differential count are granulocytes, lymphocytes, monocytes, eosinophils, and basophils.

A machine-generated percentage of the different types of white blood cells is called the automated WBC differential. These components can also be counted under the microscope on a glass slide by a trained laboratory technician or a doctor and referred to as the manual WBC differential.

  • Red cell count (RBC) signifies the number of red blood cells in a volume of blood. Normal range varies slightly between laboratories but is generally from 4.2 to 5.9 million cells/cmm. This can also be referred to as the erythrocyte count and can be expressed in international units as 4.2 to 5.9 x 1012 cells per liter.

Red blood cells are the most common cell type in blood and people have millions of them in their blood circulation. They are smaller than white blood cells, but larger than platelets.

Test tube of blood on top of lab test results.
Source: iStock

What are values for a complete blood count (mean corpuscular and platelet volume)?

Mean corpuscular volume (MCV): is the average volume of a red blood cell. This is a calculated value derived from the hematocrit and red cell count. Normal range may fall between 80 to 100 femtoliters (a fraction of one millionth of a liter).

Mean Corpuscular Hemoglobin (MCH): is the average amount of hemoglobin in the average red cell. This is a calculated value derived from the measurement of hemoglobin and the red cell count. Normal range is 27 to 32 picograms.

Mean Corpuscular Hemoglobin Concentration (MCHC): is the average concentration of hemoglobin in a given volume of red cells. This is a calculated volume derived from the hemoglobin measurement and the hematocrit. Normal range is 32% to 36%.

Mean Platelet Volume (MPV): is the average size of platelets in a volume of blood.

What are the functions of the cells in a complete blood count (CBC)?

The cells in the CBC (white blood cells, red blood cells, and platelets) have unique functions. Generally speaking, white blood cells are an essential part of the immune system and help the body fight infections. Each different component of the white blood cell (the WBC differential) plays a specific role in the immune system.

Red blood cells are essential in transporting oxygen to all the cells in the body to serve their functions. The hemoglobin molecule in the red blood cell is the vehicle for the transportation of oxygen. Platelets are a part of the blood clotting system in the body and help in preventing bleeding.

A doctor going over a patients complete blood count (CBC) results with her.
Source: iStock

What is the complete blood count (CBC) used for?

Your doctor may order this test for a variety of reasons. It may be a part of a routine check-up or screening, or as a follow-up test to monitor certain treatments. It can also be done as a part of an evaluation based on a patient's symptoms.

For example, a high WBC count (leukocytosis) may signify an infection somewhere in the body or, less commonly, it may signify an underlying malignancy. A low WBC count (leukopenia) may point toward a bone marrow problem or related to some medications, such as chemotherapy. A doctor may order the test to follow the WBC count in order to monitor the response to a treatment for an infection. The components in the differential of the WBC count also have specific functions and if altered, they may provide clues for particular conditions.

A low red blood cell count or low hemoglobin may suggest anemia, which can have many causes. Possible causes of high red blood cell count or hemoglobin (erythrocytosis) may include bone marrow disease or low blood oxygen levels (hypoxia).

A low platelet count (thrombocytopenia) may be the cause of prolonged bleeding or other medical conditions that affect the production of platelets in the bone marrow. Conversely, a high platelet count (thrombocytosis) may point toward a bone marrow problem or severe inflammation.

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Reviewed on 6/28/2018

REFERENCE:

Curry, CV, MD, et al. Differential Blood Count. Medscape. Updated: Jan 14, 2015. National Institutes of Health
<https://emedicine.medscape.com/article/2085133-overview>

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