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Complete Blood Count Test with Differential in Children

What does a CBC with differential test for?

A complete blood count test (CBC) measures the number and types of cells in the blood. A CBC test can also measure some proteins made by the blood cells.

Blood is a body fluid that has cells in a liquid called plasma, made of water and proteins. Children with infections, cancers, or blood disorders have changes in the number and kind of cells and proteins in their blood. Therapies and medications can also change their blood.

Graph showing structure of blood with red blood cells at bottom at 41%, white blood cells in middle making up about 4%, and plasma on top at 55%

Blood has red blood cells, white blood cells, and platelets that are in a fluid called plasma. The CBC measures how many of each kind are in the blood.

Blood cells have important jobs. The body has 3 main types:

  • Red blood cells that carry oxygen through the body
  • White blood cells that help fight infection and disease
  • Platelets that help form blood clots after injury

What is a differential test?

A differential test gives even more information about the white blood cells that fight infection and disease.

What CBC with differential test results show

Health care providers use the CBC with differential tests to:

  • Diagnose a medical problem
  • Check if your child has an infection
  • Tell how your child fights infection
  • Study how a disease affects their body
  • Tell if a treatment is working (monitoring)

How to prepare for a CBC test

Follow your health care provider’s instructions before the test. Tell them if your child is taking any:

  • Medicines (both prescribed and those without prescription)
  • Vitamins
  • Herbal preparations

These things may change the test results. If your child is only getting a complete blood count test (CBC), they might be able to eat and drink as normal. But if your doctor orders other blood tests, they might not be able to eat or drink.

For children with cancer, regular complete blood count tests help the care team monitor the patient’s health and ability to fight off infections. In this image, a nurse draws blood by inserting a needle into a vein and collecting the blood in attached vials.

For children with cancer, regular complete blood count tests help the care team monitor the patient’s health and ability to fight off infections.

Steps of a CBC test

A care team member inserts a needle into your child’s vein to get a blood sample and collects it in vials or tubes. They can also get blood from your child’s central venous access device such as a:

If your child does not have a device, the provider might start a peripheral IV to get blood samples.

The laboratory staff will study your child’s blood and report the results to your doctor.

What a CBC test measures

CBC tests measure:

  • Red blood cells (RBC) - The numbers, size, and types of red blood cells
  • White blood cells (WBC) - The numbers and types of white blood cells
  • Platelets - The numbers and size of the platelets
  • Hemoglobin - An iron-rich protein in red blood cells that carries oxygen to the body’s tissues
  • Hematocrit (HCT or PCV)– How much space red blood cells take up in the blood compared to plasma and other cells
  • Reticulocyte count – Number of young red blood cells in the blood
  • Mean corpuscular volume (MCV) - The average size of red blood cells

The number of cells, their size, and types in healthy person usually falls within a range of numbers (reference range or standard range). Your doctor compares your child’s results to normal reference ranges. Normal ranges can vary widely and depend on your child’s age. You should review these results with your health care provider who can explain what is normal for your child.

CBC with Differential

A complete blood count with a differential looks more closely at the types of white blood cells in the blood to study your child’s disease.

Types of White Blood Cells

Types of blood cells and related conditions

Low numbers or abnormalities of certain types of blood cells can lead to health conditions that require treatment such as anemia, blood clotting disorders, bleeding disorders, and leukopenia (low numbers of white blood cells, and blood cancers.

Key Points

  • A complete blood count test (CBC) measures the number and types of cells in the blood.
  • A CBC test can also measure some proteins made by the blood cells, such as hemoglobin.
  • A differential test gives detailed information about the types and numbers of white blood cells.
  • These tests are useful for the diagnosis and monitoring of disease and to see how well treatments are working.


Reviewed: March 2022