What is a Blood Cell Count?

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  • Written By: H. Colledge
  • Edited By: Heather Bailey
  • Last Modified Date: 24 September 2019
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A blood cell count involves measuring the amounts of different types of cells in the blood. It is useful to help detect illnesses such as anemia, where the oxygen-carrying red blood cells can be abnormal in terms of their numbers or their structure. The amount of white blood cells, which form part of the body's immune system, might increase if a person has an infection. Platelets, which are involved in blood clotting, can be decreased in diseases such as hemophilia, where bleeding is excessive. What is known as a complete, or full, cell count measures the numbers of white cells, red cells and platelets in a unit of blood, determines the proportion of red cells and how much hemoglobin they contain, and checks their size.

The complete blood count, which doctors may use to check the general health of a patient, is probably the most common type of blood count carried out. This test can be performed quite simply by using a small syringe with a needle to draw a sample of blood from a vein in the arm. The analysis and counting of the cells in the sample is generally carried out by specialized machines. Cell shapes can also be examined if part of the sample is taken to make what is called a blood smear, where a thin layer of blood is placed on a slide and analyzed under a microscope.


Many different illnesses can be detected from a complete blood count, with perhaps the one most frequently found being anemia. Anemia can be indicated if the red blood cell count is found to be low, or the amount of hemoglobin is at a subnormal level. In conditions of low oxygen, such as at high altitude, or in diseases affecting the bone marrow, the red cells may show a high blood cell count, and the amount of hemoglobin may be increased.

Where the white blood cell count is too high, an infection or, more rarely, a cancer could be the cause. Monitoring the levels of white cells can sometimes be useful to assess whether the treatment of an infection is working. Some diseases of the bone marrow may lead to the opposite finding of a low blood count for white cells, as this kind of illness can mean their production in bone marrow is affected. Bone marrow disorders can cause an increase or a decrease in the amount of platelets in the blood, and where there are too few to enable normal blood clotting, abnormal bleeding may occur.

Most abnormalities found in a blood cell count can have a number of causes. This means that, usually, further tests will need to be performed to obtain a more exact diagnosis. The treatment of any condition found will vary depending on its cause.


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