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Complete Blood Count (CBC) Test

What is a complete blood count test?

A complete blood count (CBC) is a test that measures the number and types of blood cells including red blood cells, white blood cells, and platelets. 

A CBC with differential gives details about the types and numbers of white blood cells.

A CBC can be used to help diagnose conditions such as cancer, blood disorders, infections, and other illnesses. It can also be used to give information about your child’s health, how medicines or treatments are affecting their body, and how well their immune system is working.

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%

A complete blood count (CBC) test measures the number and type of cells in the blood.

How a CBC test works

A health care provider will take a small sample of your child’s blood from a vein. The care team can take blood through your child’s central line if they have one. 

The blood sample will be sent to a lab. The time it takes to get to the results can vary. But you will usually get the report within a few days.

Your child’s CBC test report will include your child’s results and a range of numbers (called a reference range or standard range). The reference ranges are values that are considered normal for each of the measures. Your doctor compares your child’s results to these ranges. Normal ranges can vary and depend on your child’s age, sex, and other factors. 

Review your child’s results with your care team. They can explain the numbers and what they mean for your child. 

What a CBC test measures

A CBC test measures:

  • Red blood cells (RBCs): blood cells that carry oxygen throughout the body
  • White blood cells (WBCs): blood cells that help fight infection 
  • Platelets: blood cells that help the blood to clot and stop bleeding 
  • Hemoglobin:  iron-rich protein in red blood cells that carries oxygen to the body’s tissues
  • Hematocrit (HCT or PCV): how much of the blood is made up of red blood cells 
  • Mean corpuscular volume (MCV): average size of red blood cells
  • Reticulocyte count: number or percent of young red blood cells in the blood

CBC with differential

A CBC test with differential includes more information about your child's white blood cells. White blood cells are an important part of the immune system. Each type has a different job. A blood differential can give more information about your child's health.

Types of white blood cells include:

  • Neutrophil
  • Lymphocyte
  • Monocyte
  • Eosinophil
  • Basophil

Netrophils are the most common type of white blood cell. They fight off infection. Your care team may order an absolute neutrophil count (ANC) to track your child's neutrophil levels. When the ANC is below 500 (neutropenia), the risk of infection is high.

 

Benefits of a CBC test

Your care team can use the results of a CBC test to:

  • Diagnose a medical problem
  • Check if your child has an infection
  • Tell how well your child is able to fight off infection
  • Monitor the effects of a disease on the body
  • Tell if a treatment is working 

Getting ready for a CBC test

Follow your care team’s instructions before the test. If your child is having other tests, your child may need to fast (not eat or drink). In general, food does not affect CBC test results. But if your child has other blood work at the same time, it could affect those results. Talk to your care team if you have questions.

Medicines and supplements may change the test results. Let your care team know if your child is taking any:

  • Medicines (prescription or over-the-counter)
  • Vitamins
  • Herbal products

Possible risks of a CBC test

A CBC test is simple and quick. The test is generally safe but possible risks include:

  • Pain or discomfort at the needle site
  • Bleeding or bruising  
  • Feeling dizzy or faint
  • Infection

Questions to ask your care team

  • Why is my child getting a CBC test?
  • How should my child prepare for the CBC test?
  • How long will it take to get the results of the CBC test?
  • How will we get the results of the CBC test?
  • Will my child need any follow-up tests?

Key points about a CBC test

  • A complete blood count (CBC) test measures the number and types of cells in the blood.
  • A CBC with differential gives details about the types and numbers of white blood cells.
  • Blood tests are helpful to monitor health, diagnose diseases, and to see how well treatments are working.
  • Your care team can help you understand your child’s results and how they compare to the reference ranges.

—
Reviewed: August 2024

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