What is a complete blood count (CBC)


A complete blood count, often shortened to CBC, is a common blood test that measures the number, size, and health of different types of blood cells. It provides important information about how well your bone marrow is working and how your body carries oxygen, fights infection, and controls bleeding.

A CBC is often one of the first tests ordered when someone feels unwell or when doctors want a general picture of a person’s health.

What does a CBC measure?

A CBC looks at three main types of blood cells: red blood cells, white blood cells, and platelets. Each plays a different role in the body.

Red blood cells

Red blood cells carry oxygen from the lungs to the rest of the body.

The CBC measures:

  • Hemoglobin – the oxygen-carrying protein inside red blood cells.

  • Hematocrit – the percentage of blood made up of red blood cells.

  • Red blood cell count – the total number of red blood cells in the blood.

Abnormal results can help identify anemia or other red blood cell disorders.

White blood cells

White blood cells help the body fight infections and respond to inflammation. The CBC measures the total number of white blood cells and may include a breakdown of different types, called a differential.

Changes in white blood cell counts can be seen with infections, inflammation, immune system disorders, and some cancers.

Platelets

Platelets are small cell fragments that help the blood clot and stop bleeding. The CBC measures the number of platelets in the blood. Too few platelets can increase the risk of bleeding, while too many can increase the risk of clotting.

Why is a CBC performed?

A CBC is used for many reasons, including:

  • Evaluating symptoms such as fatigue, weakness, fever, or unexplained bruising.

  • Detecting conditions like anemia, infection, or bleeding disorders.

  • Monitoring chronic diseases or the effects of treatments such as chemotherapy.

  • Checking overall health during routine medical care or before surgery.

Because it provides broad information, the CBC is often used as a screening test.

How is a CBC performed?

A CBC is performed on a small blood sample, usually taken from a vein in the arm. Automated machines analyze the blood sample in a laboratory, which count and measure the cells. In some cases, a pathologist or laboratory specialist may review the results or examine a blood smear under the microscope.

What do abnormal CBC results mean?

An abnormal CBC does not always mean there is a serious problem. Many factors, including dehydration, recent illness, medications, or pregnancy, can affect results.

When results are abnormal, doctors usually interpret them along with symptoms, medical history, and other tests to determine whether further investigation is needed.

Why is a CBC important in pathology and disease diagnosis?

The CBC provides clues about conditions affecting the blood and bone marrow. It can help pathologists and doctors recognize patterns that suggest infection, inflammation, bone marrow disorders, or cancer. While a CBC cannot make a diagnosis on its own, it often guides the next steps in testing.

Questions to ask your doctor

  • Which parts of my CBC were abnormal, if any?

  • Do these results explain my symptoms?

  • Do I need repeat testing or additional blood tests?

  • Could medications or recent illness have affected my results?

  • How will my CBC be monitored over time?

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