Complete Blood Count (CBC)

Teen donating blood before surgery
A CBC test 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 your child 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 to 10,000 white blood cells per cubic millimetre of blood.

How the CBC test works

A CBC test can find leukemic blood cells, which is a clear sign of leukemia.

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 children 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).

Common Changes in Whole Blood Count for Leukemia
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In many cases, white blood cell count can be normal or decreased at diagnosis. Platelet count is rarely normal at diagnosis.
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. Tell your child that they will be poked with needles for the first initial days so that they are prepared.

The procedure

A CBC test is similar to 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 by a finger poke.

Does the CBC test confirm that your child has leukemia?

No. A CBC test looks for signs of leukemia, but it is not a firm diagnosis.

To confirm leukemia, the doctor tests your child’s bone marrow for a more detailed diagnosis and to help determine the type of treatment plan for your child.

Sue Zupanec, MN, NP Peds

Angela Punnett, MD

Johann Hitzler, MD, FRCP(C), FAAP

Oussama Abla, MD