Complete blood count (CBC)

What a CBC measures

• Red blood cells, which carry oxygen through your body.
• Hemoglobin, the protein in red blood cells that holds oxygen.
• Hematocrit, how much of your blood is made up of red blood cells.
• White blood cells, which fight infection. A CBC with differential also counts each type of white blood cell, including neutrophils.
• Platelets, which help your blood clot.

Why it’s done

A CBC gives a broad picture of your health. In hematology and oncology care, it is used to:

• Look for causes of symptoms such as tiredness, bruising or frequent infections
• Help diagnose blood conditions such as anemia
• Check how your bone marrow is responding to treatment
• Decide whether it is safe to give your next dose of chemotherapy
• Follow your counts over time after treatment

How it’s done

A small amount of blood is drawn from a vein in your arm, or from a port if you have one. It takes just a few minutes. You usually don’t need to fast for a CBC on its own, but if other tests are drawn at the same time, your care team will tell you if you need to prepare.

Understanding your results

Each part of the CBC has a normal range, and your report will show whether your results fall inside it. Normal ranges vary a little from lab to lab. Results that are a bit high or low are common and are not always a concern. Your care team will look at your numbers along with your health history, your treatment and your earlier results, and will explain what they mean for you.