A CBC counts the amount of blood cells in your child's circulating blood. In particular, it looks at red blood cells, white blood cells, and platelets. It is one of the first tests that doctors use to diagnose leukemia.
Knowing the number of leukemic cells in a child with ALL at diagnosis helps your doctor plan the treatment. This is important because the initial part of treatment will be slightly different if your child has more or less than 50,000 white blood cells per cubic millimetre of blood at the time of diagnosis. Healthy people have between 4,000 and 10,000 white blood cells per cubic millimetre of blood.
How the CBC test works
A CBC test can find leukemic blood cells, which are called blasts.
It can also detect changes in the amount of any type of blood cell. Finding any one of these changes in the blood can suggest the presence of leukemia. These include:
- neutropenia: a lower than normal amount of a type of white blood cell, called neutrophils, which makes people prone to infection.
- anemia: a lower than normal amount of red blood cells. Since there are not enough red blood cells to circulate oxygen through the body, anemia causes paleness and fatigue, which are common symptoms of leukemia.
- thrombocytopenia: a lower than normal amount of platelets in the blood. Because the blood cannot clot properly, common symptoms include bruising and bleeding.
A CBC test cannot distinguish between acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML).
Preparing for a CBC test
In the first few days of diagnosis, there can be many pokes and needles. Some children will be very anxious about needles and blood work. You may consider telling your child that they will be poked with needles for the first few days so that they are prepared.
The procedure
A CBC test is a routine blood test. To obtain a sample of your child's blood, a nurse draws blood from a vein in your child’s arm or in some cases, by a finger poke.
Does the CBC test confirm that your child has leukemia?
A CBC test looks for signs of leukemia, but it does not confirm a diagnosis. In some cases where there are many leukemic cells in the blood, a special test called flow cytometry can be used on the blood to confirm the diagnosis. However, in all cases, a bone marrow test is required for a more detailed diagnosis and to help determine the type of treatment needed.