Complete Blood Count Test (CBC)

What is the complete blood count test (CBC)?

Many blood tests measure the amount of certain chemicals or proteins in your child’s blood, but a complete blood count checks the blood cells themselves. It measures the numbers of different types of blood cells, their sizes, and their appearance. It is a very common and useful blood test.

In general, the test measures 3 main components of blood:

• Red blood cells (also called erythrocytes or RBCs). The test measures the number, size, shape, and appearance of the RBCs, and also the amount of hemoglobin in them. Hemoglobin carries oxygen from the lungs to the rest of the body. The part of the test called a hematocrit measures the percentage of your child’s blood that is red blood cells.
• White blood cells (also called leukocytes or WBCs). The total count of white cells is measured. White blood cells help the body's immune system fight infection. When the amounts of each of the different types of white blood cells are also measured, the test is called a differential. The most common types of white blood cells are:
  • neutrophils (cells that respond to stress such as bacterial infections)
  • lymphocytes (cells that increase when the body fights a viral infection)
  • eosinophil (cells that increase when a child has allergies)
• Platelets (also called thrombocytes). Platelets are not actually blood cells. They are fragments of large blood-forming cells. These fragments are essential for normal blood clotting.

Why is this test done?

The CBC test may be done to check your child’s overall health. It may also be done to check if your child has:

• anemia
• a high level of white blood cells, which is often a sign of infection.
• certain diseases

How do I prepare my child for this test?

Usually no preparation is needed for this test.

How is the test done?

For newborns, a blood sample is taken from the heel. For older children, a small amount of blood is taken with a fingerstick, or from the arm with a needle.

The blood is collected in tubes and sent to a lab. The lab measures the amounts of the different components in the sample of blood. The blood sample may also be viewed with a microscope to double check the different kinds of white blood cells.

Having this test will take just a few minutes. There is no risk of getting AIDS, hepatitis, or any other blood-borne disease from this test.

What do the test results mean?

The normal ranges for the different kinds of blood cells measured vary based on your child’s age. These ranges may vary slightly from lab to lab. Normal ranges are usually shown next to your child’s results in the lab report.

Some of the reasons your child’s red blood cell count may be higher than normal are:

• Your child hasn't had enough fluids.
• Your child has polycythemia vera, a disease that causes your child’s blood to be too thick because your child’s body is making too many red blood cells.

A red blood cell count or hemoglobin level lower than normal is called anemia. The size of the red blood cells gives an important clue to possible causes of anemia:

• Anemia with small red blood cells (called microcytic anemia) may be caused by a lack of iron, a stomach ulcer, or sickle cell anemia.
• Anemia with large red blood cells (called macrocytic anemia) may be caused by a lack of vitamins B-12 or folate.
• Anemia with normal size blood cells (called normocytic anemia) may be caused by bleeding from an injury or a very heavy menstrual period.

Some of the reasons your child’s white blood cell count may be higher than normal are:

• Your child has an infection.
• Your child has inflammation.
• Your child is taking certain medicines, such as prednisone.
• Your child has a type of cancer called leukemia.

Your child’s white blood cell count may be lower than normal if your child has a viral infection, including the common cold or is taking chemotherapy.

Your child’s platelet count may be higher than normal if your child has an autoimmune disease, such as rheumatoid arthritis or Crohn's disease. The platelet count can also increase with viral infections, but will go back to normal as your child recovers.

Some of the reasons your child’s platelet count may be lower than normal are:

• Your child is taking certain medicines, such as sulfa drugs, quinine, heparin, or chemotherapy.
• Your child has a blood infection or another serious illness.
• Your child has an autoimmune disease, such as lupus.

What if my child’s test result is not normal?

Test results are only one part of a larger picture that takes into account your child’s medical history and current health. Sometimes a test needs to be repeated to check the first result. Talk to your child’s healthcare provider about your child’s result and ask questions.

If your child’s test results are not normal, ask your child’s healthcare provider:

• if your child needs additional tests
• what kind of treatment your child might need
• when your child needs to be tested again.