Complete Blood Count (CBC) Test

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KEY POINTS

• A complete blood count (CBC) measures different types of blood cells, their sizes, and their appearance. The CBC test may be done to check your child’s overall health. It may also be done to check for anemia, infections, problems with blood clotting, or how well treatment for a disease or condition is working.
• If your child is a newborn, your child's healthcare provider makes a tiny cut in the baby's heel to get a small amount of blood to test. For older children, a small amount of blood is taken from the fingertip or from a vein in your child’s arm with a needle. The blood is collected in tubes and sent to a lab.
• Talk to your healthcare provider about what the test results mean and ask any questions you have.

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What is a complete blood count?

A complete blood count (CBC) checks the blood cells in the blood. It measures the numbers of different types of blood cells, their sizes, and their appearance. It is a 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. It also measures the amount of hemoglobin in the RBCs. Hemoglobin carries oxygen from the lungs to the rest of the body. The part of the test called a hematocrit measures the percentage of the blood that is red blood cells.
• White blood cells (also called leukocytes or WBCs). The test counts the number of white blood cells. White blood cells help your body fight infection. When each of the different types of white blood cells are counted, the test is called a CBC with differential. The most common types of white blood cells are:
  • Neutrophils (cells that respond to stress, such as bacterial infection)
  • Lymphocytes (cells that increase when the body fights a viral infection)
  • Eosinophils (cells that increase if your child has allergies)
• Platelets (also called thrombocytes). Platelets are not actually blood cells. They are fragments of large blood-forming cells. They are needed for 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 for:

• Anemia (too few red blood cells)
• Infection or other diseases
• Problems with blood clotting
• This test may also be done to see how well treatment for a disease or condition is working.

How do I prepare my child for this test?

Usually no preparation is needed for this test.

Talk to your healthcare provider if you have any questions about the test.

How is the test done?

Having this test will take just a few minutes. If your child is a newborn, your child's healthcare provider makes a tiny cut in the baby's heel to get a small amount of blood to test. For older children, a small amount of blood is taken from the fingertip or from a vein in your child’s arm with a needle. The blood is collected in tubes and sent to a lab.

Ask your child’s healthcare provider how and when you will get your child’s test results.

What do the test results mean?

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

• Your child hasn't had enough liquids and is dehydrated.
• Your child has a rare disease called polycythemia vera that causes the body to make too many red blood cells.
• Your child lives at a high altitude.
• Your child has certain genetic, kidney, or lung diseases.

A red blood cell count or hemoglobin level lower than normal is called anemia. The size of the red blood cells can help your child’s healthcare provider know what might be causing the anemia.

White blood cell count
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, which is swelling and irritation in the body caused by an infection or disease.
• 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 getting cancer treatment (chemotherapy).

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

• Your child has an autoimmune disease (a disease that causes your child’s body to attack its own tissue) such as juvenile arthritis or lupus.
• Your child has a viral infection.

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

• Your child has a blood clotting problem such as thrombocytopenia (an autoimmune disease)
• Your child is taking certain medicines such as sulfa drugs, quinine, heparin, or cancer drugs.
• 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 such as:

• If your child needs more tests
• What kind of treatment your child might need
• What lifestyle, diet, or other changes your child might need to make