Complete blood count (CBC) tests

Complete blood count (CBC) tests check the number of different types of blood cells in your blood. The main blood cells are:

• Red blood cells
• White blood cells
• Platelets

Each type plays a role in keeping your body healthy.

Knowing whether your blood has a normal number of these cells helps your doctors:

• Manage issues such as anemia and infection
• Monitor how well your body handles treatment
• Look for signs of cancer spread

How CBC tests work

In CBC testing, a healthcare professional uses a needle to take a blood sample from a vein, usually on the inside of the elbow. Sometimes the sample is taken from a vein on the wrist or back of the hand.

For people undergoing chemotherapy, blood can also be drawn through a port (a small device placed in the chest near a vein where chemo can be delivered).

After you give a blood sample, the blood cell levels are measured at the doctor’s office or in a lab.

CBC tests measure the amount of three important cells in your blood:

• White blood cells, which fight infection.
• Red blood cells, which carry oxygen to your body’s cells.
• Platelets, which help blood to clot. Clotting stops bleeding.

Who gets CBC tests?

Doctors use complete blood count tests to monitor your health for many reasons. For example, you may have had a CBC test if you were sick with fatigue or flu-like symptoms, even if you were not diagnosed with cancer.

In breast cancer, CBC tests are usually given during diagnosis and throughout treatment. People diagnosed with any stage of breast cancer will have CBC tests. This is because some cancer medicines, such as chemotherapy, radiation therapy, immunotherapy, and others, are known to lower blood cell counts.

Why are CBC tests important?

Normal blood cell counts are a big part of your body’s health. When your blood cell counts are off, it’s a sign of a problem. Here are some examples of what blood count results can show:

• Low white blood cell count: you have neutropenia, which means there may not be enough white blood cells to prevent infection.
• High white blood cell count: your body may already be fighting an infection.
• Low red blood cell count: you have anemia, which can lead to fatigue, fast heart rate, and other side effects.
• High red blood cell count: you could be at risk for a blood clot, or you may have another underlying condition.
• Low platelet count: you have thrombocytopenia, which means your blood may not clot as easily and you are at risk of bleeding and bruising more or longer, even from injuries that aren’t severe.
• High platelet count: like a high red blood cell count, high platelets can signify a risk of blood clots or other health conditions.

Your doctors will use CBC tests to make sure your counts don’t get too low. If blood counts get too low, your doctors will recommend ways to raise the counts back up.

How to manage low blood cell counts

If you get a CBC test and it shows you have low white blood cells, red blood cells, or platelets, you doctor will talk with you about ways to bring the counts back up. This can include eating more of certain foods, avoiding certain foods, or taking medicine. Some examples are below.

Low red blood cell count:

• Medicines that help the body make more red blood cells
• Iron supplements
• Vitamin B12 or folic acid supplements

These medicines and supplements may help make more red blood cells to bring your count back up. Always check with your care team before starting any over-the-counter vitamin or supplement.

Low white blood cell count:

• More tests to find the cause of your low white cell count
• Antibiotics to fight any existing infection
• Medicines to help the body make more white blood cells

Low platelet count:

• Adjusting the dose of or switching your cancer treatment
• Treating infection or other conditions, if one is found
• Treatments to stop bleeding, if needed

Learn more about testing and ways to manage side effects.