Complete Blood Count (CBC)

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A blood count is a test that shows the number and types of blood cells in our body. The number and types of blood cells are counted and recorded. The results of the test help your child’s doctor plan medical treatment.


Blood is the red fluid that circulates to all parts of the body. Blood is made up of cells and pale yellow liquid called plasma (PLAZ ma). Red blood cells (which give the blood its color), white blood cells, and platelets float in the plasma.

How a blood count is done

A sample of blood is taken either from a little stick in the finger or from a vein in the arm. If your child has a central line, the blood sample may be taken from it. The heel may be used for an infant. Using a small sample of the blood, cells are counted, using a laboratory machine. This machine takes measurements of the different types of cells. Another small drop is spread thinly on a glass slide. It is stained with different colored dyes. Each type of cell then becomes a different color. The blood on the slide is examined under a microscope. The different cells are counted.

What blood contains

Blood is made up of plasma (about 55 per cent), cells (about 45 per cent) and tiny amounts of other substances. Plasma is liquid; the cells, platelets and other substances are suspended in the plasma.

This is what blood is made of:

  • Cells and platelets (about 45%) - solid
    • Red cells
    • White cells
    • Platelets        
  • Plasma (about 55%) - liquid
    • Water
    • Blood sugar
    • Salts
    • Proteins
    • Blood fat         

All blood cells and other substances are suspended in the liquid plasma.

Red blood cells

The red blood cells are made in the bone marrow (the soft, spongy substance inside bones).

In infants less than a year old, the red blood cells are made in the bone marrow of all bones. In people 1 year of age and older, the red blood cells are made in the marrow of just the pelvic bones, sternum (breast bone), ribs and vertebrae.

Red blood cells contain hemoglobin (HE moe glow bin). The oxygen that is breathed into the lungs is carried to all parts of the body by the hemoglobin.

White blood cells

The white blood cells are made in the lymph nodes, spleen, and bone marrow of certain bones of the body. The white cells fight infection, protect the body from infectious diseases and foreign bodies by increasing in number.

There are two main types of white cells:

  • Granulocytes (GRAN u low sites) that fight bacteria
  • Lymphocytes (LIMP fo sites) that fight viruses.


Platelets are also made in the bone marrow. The platelets, along with coagulation factors, help stop bleeding if a blood vessel is cut or injured by forming a clot that helps to seal the cut.

Your child’s doctor or nurse will help you understand the results of your blood test. Be sure to ask one of them if you have questions.

For directions to the nearest Laboratory Service Center, please call Laboratory Services at (800) 934-6575 or visit

Complete Blood Count (PDF)

HH-III-58 7/81, Revised 4/17 Copyright 1981 Nationwide Children’s Hospital